Table of contents

Spider is a long-duration, balloon-borne polarimeter designed to measure large-scale cosmic microwave background (CMB) polarization with very high sensitivity and control of systematics. The instrument will map over half the sky with degree angular resolution in the I, Q, and U Stokes parameters in four frequency bands from 96 to 275 GHz. Spider's ultimate goal is to detect the primordial gravity-wave signal imprinted on the CMB B-mode polarization. One of the challenges in achieving this goal is the minimization of the contamination of B-modes by systematic effects. This paper explores a number of instrument systematics and observing strategies in order to optimize B-mode sensitivity. This is done by injecting realistic-amplitude, time-varying systematics into a set of simulated time streams. Tests of the impact of detector noise characteristics, pointing jitter, payload pendulations, polarization angle offsets, beam systematics, and receiver gain drifts are shown. Spider's default observing strategy is to spin continuously in azimuth, with polarization modulation achieved by either a rapidly spinning half-wave plate or a rapidly spinning gondola and a slowly stepped half-wave plate. Although the latter is more susceptible to systematics, the results shown here indicate that either mode of operation can be used by Spider.

We use Swift BAT Earth occultation data at different geomagnetic latitudes to derive a sensitive measurement of the cosmic X-ray background (CXB) and of the Earth albedo emission in the 15-200 keV band. We compare our CXB spectrum with recent (INTEGRAL, BeppoSAX) and past results (HEAO-1) and find good agreement. Using an independent measurement of the CXB spectrum we are able to confirm our results. This study shows that the BAT CXB spectrum has a normalization ~8% ± 3% larger than the HEAO-1 measurement. The BAT accurate Earth albedo spectrum can be used to predict the level of photon background for satellites in low Earth and mid inclination orbits.

Polar ring galaxies, such as NGC 4650A, are a class of galaxies that have two kinematically distinct components that are inclined by almost 90° to each other. These striking galaxies challenge our understanding of how galaxies form; the origin of their distinct components has remained uncertain and is the subject of much debate. We use high-resolution cosmological simulations of galaxy formation to show that polar ring galaxies are simply an extreme example of the misalignment of angular momentum that occurs during the hierarchical structure formation characteristic of a cold dark matter cosmology. In our model, polar ring galaxies form through the continuous accretion of gas whose angular momentum is misaligned with the central galaxy.

We present a table of redshifts for 2907 galaxies and stars in the 145 arcmin²HST ACS GOODS-North, making this the most spectroscopically complete redshift sample obtained to date in a field of this size. We also include the redshifts, where available, in a table containing just under 7000 galaxies from the ACS area with K_{s, AB} < 24.5 measured from a deep K_s image obtained with WIRCam on the CFHT, as well as in a table containing 1016 sources with NUV_AB < 25 and 478 sources with FUV_AB < 25.5 (there is considerable overlap) measured from the deep GALEX images in the ACS area. Finally, we include the redshifts, where available, in a table containing the 1199 24 μm sources to 80 μJy measured from the wider area Spitzer GOODS-North. The redshift identifications are greater than 90% complete to magnitudes of F 435W_AB = 24.5, F 850LP_AB = 23.3, and K_{s, AB} = 21.5, and to 24 μm fluxes of 250 μJy. An extensive analysis of these data appear in a companion paper, but here we test the efficiency of color-selection techniques to identify populations of high-redshift galaxies and active galactic nuclei. We also examine the feasibility of doing tomography of the intergalactic medium with a 30 m telescope.

Photometric redshift (photo-z) estimates are playing an increasingly important role in extragalactic astronomy and cosmology. Crucial to many photo-z applications is the accurate quantification of photometric redshift errors and their distributions, including identification of likely catastrophic failures in photo-z estimates. We consider several methods of estimating photo-z errors, and propose new training-set based error estimators based on spectroscopic training set data. Using data from the Sloan Digital Sky Survey and simulations of the Dark Energy Survey as examples, we show that this method provides a robust, relatively unbiased estimate of photo-z errors. We show that culling objects with large, accurately estimated photo-z errors from a sample can reduce the incidence of catastrophic photo-z failures.

We have determined the distance to NGC 4258 using observations made with the Hubble Space Telescope (HST) and the Wide Field, Advanced Camera for Surveys (ACS WFC). We apply a modified technique that fully accounts for metallicity effects on the use of the luminosity of the tip of the red giant branch (TRGB) to determine one of the most precise TRGB distance moduli to date, μ (TRGB) = 29.28 ± 0.04 (random) ± 0.12 (systematic) mag (7.18 ± 0.13 ± 0.40 Mpc). We discuss this distance modulus with respect to other recent applications of the TRGB method to NGC 4258 and with several other techniques (Cepheids and masers) that are equally competitive in their precision, but different in their systematics.

Growth of massive black holes (MBHs) in galactic centers comes mainly from gas accretion during their QSO/AGN phases. In this paper we apply an extended Sołtan argument, connecting the local MBH mass function with the time integral of the QSO luminosity function to the demography of MBHs and QSOs from recent optical and X-ray surveys, and obtain robust constraints on the luminosity evolution (or mass growth history) of individual QSOs (or MBHs). We find that the luminosity evolution probably involves two phases, an initial exponentially increasing phase set by the Eddington limit and a following phase in which the luminosity declines with time as a power law (with a slope of ~–1.2 to –1.3) set by a self-similar long-term evolution of disk accretion. Neither an evolution involving only the increasing phase with a single Eddington ratio nor an exponentially declining pattern in the second phase is likely. The period of a QSO radiating at a luminosity higher than 10% of its peak value is about (2–3) × 10⁸ yr, during which the MBH obtains ~80% of its mass. The mass-to-energy conversion efficiency is ≃0.16 ± 0.04^{+ 0.05}₋₀, with the latter error accounting for the maximum uncertainty due to Compton-thick AGNs. The expected Eddington ratios in QSOs from the constrained luminosity evolution cluster around a single value close to 0.5-1 for high-luminosity QSOs and extend to a wide range of lower values for low-luminosity ones. The Eddington ratios for high-luminosity QSOs appear to conflict with those estimated from observations (~0.25) by using some virial mass estimators for MBHs in QSOs, unless the estimators systematically overestimate MBH masses by a factor of 2-4. We also infer the fraction of optically obscured QSOs, ~60%-80%. The constraints obtained above are not affected significantly by MBH mergers and multiple times of nuclear activity (e.g., triggered by multiple times of galaxy wet major mergers) in the MBH growth history. We discuss further applications of the luminosity evolution of individual QSOs to obtaining the MBH mass function at high redshifts and the cosmic evolution of triggering rates of nuclear activity.

We analyzed the microlensing of the X-ray and optical emission of the lensed quasar PG 1115+080. We find that the effective radius of the X-ray emission is 1.3^{+ 1.1}_−0.5 dex smaller than that of the optical emission. Viewed as a thin disk observed at inclination angle i, the optical accretion disk has a scale length, defined by the point where the disk temperature matches the rest-frame energy of the monitoring band (kT = hc/λ_rest with λ_rest = 0.3 μm), of log{(r_{s, opt}/cm)[cos(i)/0.5]^½} = 16.6 ± 0.4. The X-ray emission region (1.4-21.8 keV in the rest frame) has an effective half-light radius of log (r_1/2,X/cm) = 15.6^{+ 0.6}_−0.9. Given an estimated black hole mass of 1.2 × 10⁹M_☉, corresponding to a gravitational radius of log (r_g/cm) = 14.3, the X-ray emission is generated near the inner edge of the disk, while the optical emission comes from scales slightly larger than those expected for an Eddington-limited thin disk. We find a weak trend supporting models with low stellar mass fractions near the lensed images, in mild contradiction to inferences from the stellar velocity dispersion and the time delays.

We present a systematic X-ray study of eight active galactic nuclei (AGNs) with intermediate-mass black holes (M_BH ∼ 8–95 × 10⁴M_☉) based on 12 XMM-Newton observations. The sample includes the two prototype AGNs in this class—NGC 4395 and POX 52 and six other AGNs discovered with the Sloan Digitized Sky Survey. These AGNs show some of the strongest X-ray variability, with the normalized excess variances being the largest and the power density break timescales being the shortest observed among radio-quiet AGNs. The excess-variance-luminosity correlation appears to depend on both the BH mass and the Eddington luminosity ratio. The break timescale-black hole mass relations for AGN with IMBHs are consistent with that observed for massive AGNs. We find that the FWHM of the Hβ/Hα line is uncorrelated with the BH mass, but shows strong anticorrelation with the Eddington luminosity ratio. Four AGNs show clear evidence for soft X-ray excess emission (kT_in ∼ 150–200 eV). X-ray spectra of three other AGNs are consistent with the presence of the soft excess emission. NGC 4395 with lowest L/L_Edd lacks the soft excess emission. Evidently small black mass is not the primary driver of strong soft X-ray excess emission from AGNs. The X-ray spectral properties and optical-to-X-ray spectral energy distributions of these AGNs are similar to those of Seyfert 1 galaxies. The observed X-ray/UV properties of AGNs with IMBHs are consistent with these AGNs being low-mass extensions of more massive AGNs, those with high Eddington luminosity ratio looking more like narrow-line Seyfert 1 s and those with low L/L_Edd looking more like broad-line Seyfert 1 galaxies.

Supermassive black holes in giant elliptical galaxies are remarkably faint given their expected accretion rates. This motivates models of radiatively inefficient accretion due to either ion-electron thermal decoupling, generation of outflows that inhibit accretion, or settling of gas to a gravitationally unstable disk that forms stars in preference to feeding the black hole. The latter model predicts the presence of cold molecular gas in a thin disk around the black hole. Here we report Submillimeter Array observations of the nucleus of the giant elliptical galaxy M87 that probe 230 GHz continuum and CO (J = 2–1) line emission. Continuum emission is detected from the nucleus and several knots in the jet, including one that has been undergoing flaring behavior. We estimate a conservative upper limit on the mass of molecular gas within ~100 pc and ±400 km s⁻¹ line-of-sight velocity of the central black hole of ~8 × 10⁶M_☉, which includes an allowance for possible systematic errors associated with subtraction of the continuum. Ignoring such errors, we have a 3 σ sensitivity to ~3 × 10⁶M_☉. In fact, the continuum-subtracted spectrum shows weak emission features extending up to 4 σ above the rms dispersion of the line-free channels. These may be artifacts of the continuum subtraction process. Alternatively, if they are interpreted as CO emission, then the implied molecular gas mass is ~5 × 10⁶M_☉ spread out over a velocity range of 700 km s⁻¹. These constraints on molecular gas mass are close to the predictions of the model of self-gravitating, star-forming accretion disks fed by Bondi accretion (Tan & Blackman 2005).

We present an analysis of the chemical and ionization conditions in a sample of 100 weak Mg II absorbers identified in the VLT/UVES archive of quasar spectra. In addition to Mg II, we present equivalent width and column density measurements of other low ionization species such as Mg I, Fe II, Al II, C II, Si II, and also Al III. We find that the column densities of C II and Si II are strongly correlated with the column density of Mg II, with minimal scatter in the relationships. The column densities of Fe II exhibit an appreciable scatter when compared with the column density of Mg II, with some fraction of clouds having N(Fe II) ∼ N(Mg II), in which case the density is constrained to n_H > 0.05 cm⁻³. Other clouds in which N(Fe II) ≪ N(Mg II) have much lower densities. From ionization models, we infer that the metallicity in a significant fraction of weak Mg II clouds is constrained to values of solar or higher, if they are sub-Lyman-limit systems. Based on the observed constraints, we hypothesize that weak Mg II absorbers are predominantly tracing two different astrophysical processes/structures. A significant population of weak Mg II clouds, those in which N(Fe II) ≪ N(Mg II), identified at both low (z ∼ 1) and high (z ∼ 2) redshift, are likely to be tracing gas in the extended halos of galaxies, analogous to the Galactic high-velocity clouds. These absorbers might correspond to α-enhanced interstellar gas expelled from star-forming galaxies, in correlated supernova events. The N(Mg II) and N(Fe II)/N(Mg II) in such clouds are also closely comparable to those measured for the high-velocity components in strong Mg II systems. An evolution is found in N(Fe II)/N(Mg II) from z = 2.4 to z = 0.4, with an absence of weak Mg II clouds with N(Fe II) ∼ N(Mg II) at high-z. The N(Fe II) ∼ N(Mg II) clouds, which are prevalent at lower redshifts (z < 1.5), must be tracing Type Ia enriched gas in small, high-metallicity pockets in dwarf galaxies, tidal debris, or other intergalactic structures.

Our recent determination of a Salpeter slope for the initial mass function (IMF) in the field of 30 Doradus appears to be in conflict with simple probabilistic counting arguments advanced in the past to support observational claims of a steeper IMF in the LMC field. In this paper we reexamine these arguments and show by explicit construction that, contrary to these claims, the field IMF is expected to be exactly the same as the stellar IMF of the clusters out of which the field was presumably formed. We show that the current data on the mass distribution of clusters themselves is in excellent agreement with our model, and is consistent with a single spectrum by the number of stars of the type n^β, with β between –1.8 and –2.2 down to the smallest clusters, without any preferred mass scale for cluster formation. We also use the random sampling model to estimate the statistics of the maximal mass star in clusters, and confirm the discrepancy with observations found by Weidner & Kroupa. We argue that rather than signaling the violation of the random sampling model, these observations reflect the gravitationally unstable nature of systems with one very large mass star. We stress the importance of the random sampling model as a null hypothesis, whose violation would signal the presence of interesting physics.

Central peaks in the iron abundance of intracluster plasma are a common feature of cooling-core galaxy clusters. Although they are centrally localized, these abundance peaks have a broader profile than the stars of the brightest cluster galaxy (BCG) that produce the excess iron, indicating that metal-enriched plasma is transported out of the BCG by some process such as turbulent diffusion. The AGN-driven convection model of Chandran & Rasera predicts the turbulent velocity profile in a steady state cluster in which radiative cooling is balanced by heating from AGN-driven convection and thermal conduction. We use the velocity profiles from this model as input into an advection/diffusion model for the transport of metals in the intracluster medium. We compare the results of our model to XMM-Newton and Chandra observations of eight clusters. Assuming a constant turbulence level over a cluster's lifetime equal to the present value, the turbulent velocities in the model can explain the observed abundance profiles in only five of the eight clusters. However, we go on to develop an analytic fit of the turbulent velocity profile as a function of the AGN power. We then deduce for each cluster the average AGN power (during the past ~10 Gyr) required to match the abundance profiles. The required average values are between 10⁴³ and 2 × 10⁴⁴ erg s⁻¹, while the present AGN powers span a much larger range. Our results suggest that AGN-driven convection can account for the observed abundance profiles if the AGN power varies in time with average values in the above-quoted range.

We present a systematic analysis of XMM-Newton observations of eight cool-core clusters of galaxies and determine the Fe distribution in the intracluster medium relative to the stellar distribution in the central dominant galaxy (CDG). Our analysis shows that the Fe is significantly more extended than the stellar mass in the CDG in all of the clusters in our sample, with a slight trend of increasing extent with increasing central cooling time. The excess Fe within the central 100 kpc in these clusters can be produced by Type Ia supernovae from the CDG over the past 3-7 Gyr. Since the excess Fe primarily originates from the CDG, it is a useful probe for determining the motion of the gas and the mechanical energy deposited by AGN outbursts over the past ~5 Gyr in the centers of clusters. We explore two possible mechanisms for producing the greater extent of the Fe relative to the stars in the CDG, bulk expansion of the gas and turbulent diffusion of the Fe. Assuming that the gas and Fe expand together, we find that a total energy of 10⁶⁰-10⁶¹ erg s⁻¹ must have been deposited into the central 100 kpc of these clusters in order to produce the currently observed Fe distributions. Since the required enrichment time for the excess Fe is approximately 5 Gyr in these clusters, this gives an average AGN mechanical power over this time of 10⁴³-10⁴⁴ erg s⁻¹. The extended Fe distribution in cluster cores can also arise from turbulent diffusion. Assuming a steady state (i.e., the outward mass flux of Fe across a given surface is equal to the mass injection rate of Fe within that surface), we find that diffusion coefficients of 10²⁹-10³⁰ cm² s⁻¹ are required in order to maintain the currently observed Fe profiles. We find that heating by both turbulent diffusion of entropy and dissipation are important heating mechanisms in cluster cores. In half of the clusters with central cooling times greater than 1 Gyr, we find that heating by turbulent diffusion of entropy alone can balance radiative losses. In the remaining clusters, some additional heating by turbulent dissipation, with turbulent velocities of 150-300 km s⁻¹, is required in order to balance radiative cooling. We also find that the average Type Ia supernova fraction within the central 100 kpc of these clusters is 0.53 (roughly twice the solar value), on the basis of the Si-to-Fe mass ratio. This implies a total (Type Ia plus core-collapse) supernova heating rate of less than 10% of the bolometric X-ray luminosity within the centers of clusters.

We have studied the abundance of oxygen in the IGM by analyzing O VI, C IV, Si IV, and H I pixel optical depths derived from a set of high-quality VLT and Keck spectra of 17 QSOs at 2.1≲ z≲ 3.6. Comparing ratios τ_{O VI}/τ_{C IV}(τ_{C IV}) to those in realistic, synthetic spectra drawn from a hydrodynamical simulation and comparing to existing constraints on [Si/C] places strong constraints on the ultraviolet background (UVB) model using weak priors on allowed values of [Si/O]: for example, a quasar-only background yields [ Si/O ] ≈ 1.4, which is highly inconsistent with the [ Si/O ] ≈ 0 expected from nucleosynthetic yields and with observations of metal-poor stars. Assuming a fiducial quasar+galaxy UVB consistent with these constraints yields a primary result that [ O/C ] = 0.66 ± 0.06 ± 0.2; this result pertains to gas with overdensity δ ≳ 2. Consistent results are obtained by similarly comparing τ_{O VI}/τ_{H I}(τ_{H I}) and τ_{O VI}/τ_{Si IV}(τ_{Si IV}) to simulation values, and also by directly ionization-correcting τ_{O VI}/τ_{H I} as a function of τ_{H I} into [O/H] as a function of density. Subdividing the sample reveals no evidence for evolution, but low- and high-τ_{H I} samples are inconsistent, suggesting either density dependence of [O/C] or—more likely—prevalence of collisionally ionized gas at high density.

In this series of papers, we study the structure of the atomic-to-molecular transition in the giant atomic-molecular complexes that are the repositories of most molecular gas in galaxies, with the ultimate goal of attaining a better understanding of what determines galaxies' molecular content. Here we derive an approximate analytic solution for the structure of a photodissociation region (PDR) in a cloud of finite size that is bathed in an external dissociating radiation field. Our solution extends previous work, which with few exceptions has been restricted to a one-dimensional treatment of the radiation field. We show that our analytic results compare favorably to exact numerical calculations in the one-dimensional limit. However, our more general geometry provides a more realistic representation than a semi-infinite slab for atomic-molecular complexes exposed to the interstellar radiation field, particularly in environments such as low-metallicity dwarf galaxies, where the curvature and finite size of the atomic envelope cannot be neglected. For clouds that are at least 20% molecular, we obtain analytic expressions for the molecular fraction in terms of properties of the gas and radiation field that are accurate to tens of percent, while for clouds of lower molecular content we obtain upper limits. As a side benefit, our analysis helps to clarify when self-shielding is the dominant process in H₂ formation, and under what circumstances shielding by dust makes a significant contribution.

We present an analysis of the radio properties of large samples of Lyman break galaxies (LBGs) at z ∼ 3, 4, and 5 from the COSMOS field. The median stacking analysis yields a statistical detection of the z ∼ 3 LBGs (U-band dropouts), with a 1.4 GHz flux density of 0.90 ± 0.21 μJy. The stacked emission is unresolved, with a size <1, or a physical size <8 kpc. The total star formation rate implied by this radio luminosity is 31 ± 7 M_☉ yr⁻¹, based on the radio-FIR correlation in low-redshift star-forming galaxies. The star formation rate derived from a similar analysis of the UV luminosities is 17 M_☉ yr⁻¹, without any correction for UV dust attenuation. The simplest conclusion is that the dust attenuation factor is 1.8 at UV wavelengths. However, this factor is considerably smaller than the standard attenuation factor of ~5, normally assumed for LBGs. We discuss potential reasons for this discrepancy, including the possibility that the dust attenuation factor at z ⩾ 3 is smaller than at lower redshifts. Conversely, the radio luminosity for a given star formation rate may be systematically lower at very high redshift. Two possible causes for a suppressed radio luminosity are (1) increased inverse Compton cooling of the relativistic electron population due to scattering off the increasing CMB at high redshift or (2) cosmic-ray diffusion from systematically smaller galaxies. The radio detections of individual sources are consistent with a radio-loud AGN fraction of 0.3%. One source is identified as a very dusty, extreme starburst galaxy (a "submillimeter galaxy").

High-redshift, dust-obscured galaxies, selected to be luminous in the radio but relatively faint at 850 μm, appear to represent a different population from the ultraluminous submillimeter-bright population. They may be star-forming galaxies with hotter dust temperatures, or they may have lower far-infrared luminosities and larger contributions from obscured active galactic nuclei (AGNs). Here we present observations of three z ∼ 2 examples of this population, which we term "submillimeter-faint radio galaxies" (SFRGs; RG J163655, RG J131236, and RG J123711) in CO(3-2) using the IRAM Plateau de Bure Interferometer to study their gas and dynamical properties. We estimate the molecular gas mass in each of the three SFRGs (8.3 × 10⁹, <5.6 × 10⁹, and 15.4 × 10⁹M_☉, respectively) and, in the case of RG J163655, a dynamical mass by measurement of the width of the CO(3-2) line (8 × 10¹⁰csc²iM_☉). While these gas masses are substantial, on average they are 4 times lower than submillimeter-selected galaxies (SMGs). Radio-inferred star formation rates (<SFR_radio > = 970 M_☉ yr⁻¹) suggest much higher star formation efficiencies than are found for SMGs and shorter gas depletion timescales (~11 Myr), much shorter than the time required to form their current stellar masses (~160 Myr; ~10¹¹M_☉). By contrast, star formation rates (SFRs) may be overestimated by factors of a few, bringing the efficiencies in line with those typically measured for other ultraluminous star-forming galaxies and suggesting that SFRGs are more like ultraviolet-selected (UV-selected) star-forming galaxies with enhanced radio emission. A tentative detection of RG J163655 at 350 μm suggests hotter dust temperatures, and thus gas-to-dust mass fractions, similar to the SMGs.

We present new Spitzer, UKIRT, and MMT observations of the blue compact dwarf galaxy (BCD) Mrk 996, with an oxygen abundance of 12 + log (O/H) = 8.0. This galaxy possesses an extraordinarily dense nuclear star-forming region, with a central density of ~10⁶ cm⁻³, a very red color J − K = 1.8, broad- and narrow-line components, and ionizing radiation as hard as 54.9 eV, as implied by the presence of the [O IV] 25.89 μm line. The infrared morphology of Mrk 996 changes with wavelength, showing extended stellar photospheric emission at 4.5 μm, extended polycyclic aromatic hydrocarbon (PAH) emission at 8.0 μm, and cool extended dust emission at 160 μm. The IRS spectrum shows strong narrow PAH emission, with narrow-line widths and equivalent widths that are high for the metallicity of Mrk 996. Gaseous nebular fine-structure lines are also seen. A CLOUDY model that accounts for both the optical and mid-infrared (MIR) lines requires that they originate in two distinct H II regions: a very dense H II region where most of the optical lines arise, with densities declining from ~10⁶ at the center to a few hundred cm⁻³ at the outer radius of ~580 pc, and a H II region with a density of ~300 cm⁻³ that is hidden in the optical but seen in the MIR. The infrared lines arise mainly in the optically obscured H II region, while they are strongly suppressed by collisional deexcitation in the optically visible one. The hard ionizing radiation needed to account for the [O IV] 25.89 μm line is most likely due to fast radiative shocks propagating in a dense interstellar medium.

We derive the fundamental plane (FP) relation for a sample of 1430 early-type galaxies in the optical (r band) and the near-infrared (K band), by combining SDSS and UKIDSS data. With such a large, homogeneous data set, we are able to assess the dependence of the FP on the wave band. Our analysis indicates that the FP of luminous early-type galaxies is essentially wave band-independent, with its coefficients increasing at most by 8% from the optical to the NIR. This finding fits well into a consistent picture in which the tilt of the FP is not driven by stellar populations but results from other effects, such as nonhomology. In this framework, the optical and NIR FPs require more massive galaxies to be slightly more metal-rich than less massive ones, and to have highly synchronized ages, with an age variation per decade in mass smaller than a few percent.

We test the hypothesis that metal-poor globular clusters form within disk galaxies at redshifts z > 3. We calculate the orbits of model clusters in the time-variable gravitational potential of a Milky Way-sized galaxy, using the outputs of a cosmological N-body simulation. We find that at present the orbits are isotropic in the inner 50 kpc of the Galaxy and preferentially radial at larger distances. All clusters located outside 10 kpc from the center formed in satellite galaxies, some of which are now tidally disrupted and some of which survive as dwarf galaxies. Mergers of the progenitors lead to a spheroidal spatial distribution of model clusters, although it is more extended than that of Galactic metal-poor clusters and has a somewhat shallower power-law slope of the number density profile, γ ≈ 2.7. The combination of two-body relaxation, tidal shocks, and stellar evolution drives the evolution of the cluster mass function from an initial power law to a peaked distribution, in agreement with observations. However, not all initial conditions and not all evolution scenarios are consistent with the observed mass function of the Galactic globular clusters. We find that our best-fitting models require the average cluster density, M/R³_h, to be constant initially for clusters of all mass and to remain constant with time. However, these models do not explain the observed decrease of the mean density with galactocentric distance. Both synchronous formation of all clusters at a single epoch (z = 4) and continuous formation over a span of 1.6 Gyr (between z = 9 and 3) are consistent with the data. For both formation scenarios, we provide online catalogs of the main physical properties of model clusters.

This paper explores the mapping between the observable properties of a stellar halo in phase and abundance space and the parent galaxy's accretion history in terms of the characteristic epoch of accretion and mass and orbits of progenitor objects. The study utilizes a suite of 11 stellar halo models constructed within the context of a standard ΛCDM cosmology. The results demonstrate that coordinate-space studies are sensitive to the recent (0-8 Gyr ago) merger histories of galaxies (this timescale corresponds to the last few percent to tens of percent of mass accretion for a Milky Way-type galaxy). Specifically, the frequency, sky coverage, and fraction of stars in substructures in the stellar halo as a function of surface brightness are indicators of the importance of recent merging and of the luminosity function of infalling dwarfs. The morphology of features serves as a guide to the orbital distribution of those dwarfs. Constraints on the earlier merger history (>8 Gyr ago) can be gleaned from the abundance patterns in halo stars: within our models, dramatic differences in the dominant epoch of accretion or luminosity function of progenitor objects leave clear signatures in the [α/Fe] and [Fe/H] distributions of the stellar halo; halos dominated by very early accretion have higher average [α/Fe], while those dominated by high-luminosity satellites have higher [Fe/H]. This insight can be applied to reconstruct much about the merger histories of nearby galaxies from current and future data sets.

The halo of M31 shows a wealth of substructures, some of which are consistent with assembly from satellite accretion. Here we report on kinematic and abundance results from Keck DEIMOS spectroscopy in the near-infrared calcium triplet region of over 3500 red giant star candidates along the minor axis and in off-axis spheroid fields of M31. These data reach out to large radial distances of about 160 kpc. The derived radial velocity distributions show an indication of a kinematically cold substructure around ~17 kpc, which has been reported before. We devise a new and improved method to measure spectroscopic metallicities from the calcium triplet in low signal-to-noise ratio spectra using a weighted co-addition of the individual lines. The resulting distribution (accurate to ~0.3 dex down to signal-to-noise ratios of 5) leads us to note an even stronger gradient in the abundance distribution along M31's minor axis and in particular toward the outer halo fields than previously detected. The mean metallicity in the outer fields reaches below –2 dex, with individual values as low as ≲–2.6 dex. This is the first time such a metal-poor halo has been detected in M31. In the fields toward the inner spheroid, we find a sharp decline of ~0.5 dex in metallicity in a region at ~20 kpc, which roughly coincides with the edge of an extended disk, previously detected from star count maps. A large fraction of red giants that we detect in the most distant fields are likely members of M33's overlapping halo. A comparison of our velocities with those predicted by new N-body simulations argues that the event responsible for the Giant Stream is most likely not responsible for the full population of the inner halo. We show further that the abundance distribution of the Stream is different from that of the inner halo, from which it becomes evident, in turn, that the merger event that formed the Stream and the outer halo cannot have contributed any significant material to the inner spheroid. All these severe structure changes in the halo suggest a high degree of infall and stochastic abundance accretion governing the buildup of M31's inner and outer halo.

We present a Chandra survey of LMXBs in 24 early-type galaxies. Correcting for detection incompleteness, the X-ray luminosity function (XLF) of each galaxy is consistent with a power law with negative logarithmic differential slope, β ∼ 2.0. However, β strongly correlates with incompleteness, indicating the XLF flattens at low-L_X. The composite XLF is well fitted by a power law with a break at (2.21^{+ 0.65}_−0.56) × 10³⁸ ergs s⁻¹ and β = 1.40^{+ 0.10}_−0.13 and 2.84^{+ 0.39}_−0.30 below and above it, respectively. The break is close to the Eddington limit for a 1.4 M_☉ neutron star, but the XLF shape rules out its representing the division between neutron star and black hole systems. Although the XLFs are similar, we find evidence of some variation between galaxies. The high-L_X XLF slope does not correlate with age, but may correlate with [α/Fe]. Considering only LMXBs with L_X > 10³⁷ ergs s⁻¹, matching the LMXBs with globular clusters (GCs) identified in HST observations of 19 of the galaxies, we find the probability a GC hosts an LMXB is proportional to L^α_GCZ^γ_Fe where α = 1.01 ± 0.19 and γ = 0.33 ± 0.11. Correcting for GC luminosity and color effects, and detection incompleteness, we find no evidence that the fraction of LMXBs with L_X > 10³⁷ ergs s⁻¹ in GCs (40%), or the fraction of GCs hosting LMXBs (~6.5%) varies between galaxies. The spatial distribution of LMXBs resembles that of GCs, and the specific frequency of LMXBs is proportional to the GC specific luminosity, consistent with the hypothesis that all LMXBs form in GCs. If the LMXB lifetime is τ_L and the duty cycle is F_d, our results imply ~1.5(τ_L/10⁸ yr )⁻¹F⁻¹_d LMXBs are formed per gigayear per GC, and we place an upper limit of one active LMXB in the field per 3.4 × 10⁹L_☉ of V-band luminosity.

Spherical models of collisionless but quasi-relaxed stellar systems have long been studied as a natural framework for the description of globular clusters. Here we consider the construction of self-consistent models under the same physical conditions, but including explicitly the ingredients that lead to departures from spherical symmetry. In particular, we focus on the effects of the tidal field associated with the hosting galaxy. We then take a stellar system on a circular orbit inside a galaxy represented as a "frozen" external field. The equilibrium distribution function is obtained from the one describing the spherical case by replacing the energy integral with the relevant Jacobi integral in the presence of the external tidal field. Then the construction of the model requires the investigation of a singular perturbation problem for an elliptic partial differential equation with a free boundary, for which we provide a method of solution to any desired order, with explicit solutions to 2 orders. We outline the relevant parameter space, thus opening the way to a systematic study of the properties of a two-parameter family of physically justified nonspherical models of quasi-relaxed stellar systems. The general method developed here can also be used to construct models for which the nonspherical shape is due to internal rotation. Eventually, the models will be a useful tool to investigate whether the shapes of globular clusters are primarily determined by internal rotation, by external tides, or by pressure anisotropy.

We present abundances of C, N, O, F, Na, and Fe in six giant stars of the tidally disrupted globular cluster NGC 6712. The abundances were derived by comparing synthetic spectra with high-resolution infrared spectra obtained with the Phoenix spectrograph on the Gemini South telescope. We find large star-to-star abundance variations of the elements C, N, O, F, and Na. NGC 6712 and M4 are the only globular clusters in which F has been measured in more than two stars, and both clusters reveal F abundance variations whose amplitude is comparable to or exceeds that of O, a pattern which may be produced in M≳ 5 M_☉ AGB stars. Within the limited samples, the F abundance in globular clusters is lower than in field and bulge stars at the same metallicity. NGC 6712 and Pal 5 are tidally disrupted globular clusters whose red giant members exhibit O and Na abundance variations not seen in comparable metallicity field stars. Therefore, globular clusters such as NGC 6712 and Pal 5 cannot contribute many field stars and/or field stars do not form in environments with chemical enrichment histories like those of NGC 6712 and Pal 5. Although our sample size is small, from the amplitude of the O and Na abundance variations we infer a large initial cluster mass and tentatively confirm that NGC 6712 was once one of the most massive globular clusters in our Galaxy.

We present a comprehensive abundance analysis of 27 heavy elements in bright giant stars of the globular clusters M4 and M5 based on high-resolution, high signal-to-noise ratio spectra obtained with the Magellan Clay Telescope. We confirm and expand on previous results for these clusters by showing that (1) all elements heavier than, and including, Si have constant abundances within each cluster, (2) the elements from Ca to Ni have indistinguishable compositions in M4 and M5, (3) Si, Cu, Zn, and all s-process elements are approximately 0.3 dex overabundant in M4 relative to M5, and (4) the r-process elements Sm, Eu, Gd, and Th are slightly overabundant in M5 relative to M4. The cluster-to-cluster abundance differences for Cu and Zn are intriguing, especially in light of their uncertain nucleosynthetic origins. We confirm that stars other than Type Ia supernovae must produce significant amounts of Cu and Zn at or below the clusters' metallicities. If intermediate-mass AGB stars or massive stars are responsible for the Cu and Zn enhancements in M4, the similar [Rb/Zr] ratios and (preliminary) Mg isotope ratios in both clusters may be problematic for either scenario. For the elements from Ba to Hf, we assume that the s- and r-process contributions are scaled versions of the solar s- and r-process abundances. We quantify the relative fractions of s- and r-process material for each cluster and show that they provide an excellent fit to the observed abundances.

We report new precision measurements of the properties of our Galaxy's supermassive black hole. Based on astrometric (1995-2007) and radial velocity (RV; 2000-2007) measurements from the W. M. Keck 10 m telescopes, a fully unconstrained Keplerian orbit for the short-period star S0-2 provides values for the distance (R₀) of 8.0 ± 0.6 kpc, the enclosed mass (M_bh) of 4.1 ± 0.6 × 10⁶M_☉, and the black hole's RV, which is consistent with zero with 30 km s⁻¹ uncertainty. If the black hole is assumed to be at rest with respect to the Galaxy (e.g., has no massive companion to induce motion), we can further constrain the fit, obtaining R₀ = 8.4 ± 0.4 kpc and M_bh = 4.5 ± 0.4 × 10⁶M_☉. More complex models constrain the extended dark mass distribution to be less than 3-4 × 10⁵M_☉ within 0.01 pc, ~100 times higher than predictions from stellar and stellar remnant models. For all models, we identify transient astrometric shifts from source confusion (up to 5 times the astrometric error) and the assumptions regarding the black hole's radial motion as previously unrecognized limitations on orbital accuracy and the usefulness of fainter stars. Future astrometric and RV observations will remedy these effects. Our estimates of R₀ and the Galaxy's local rotation speed, which it is derived from combining R₀ with the apparent proper motion of Sgr A*, (θ₀ = 229 ± 18 km s⁻¹), are compatible with measurements made using other methods. The increased black hole mass found in this study, compared to that determined using projected mass estimators, implies a longer period for the innermost stable orbit, longer resonant relaxation timescales for stars in the vicinity of the black hole and a better agreement with the M_bh-σ relation.

We present new results characterizing cosmological shocks within adaptive mesh refinement N-body/hydrodynamic simulations that are used to predict nonthermal components of large-scale structure. This represents the first study of shocks using adaptive mesh refinement. We propose a modified algorithm for finding shocks from those used on unigrid simulations that reduces the shock frequency of low Mach number shocks by a factor of ~3. We then apply our new technique to a large, (512 h⁻¹ Mpc)³, cosmological volume and study the shock Mach number () distribution as a function of preshock temperature, density, and redshift. Because of the large volume of the simulation, we have superb statistics that result from having thousands of galaxy clusters. We find that the Mach number evolution can be interpreted as a method to visualize large-scale structure formation. Shocks with < 5 typically trace mergers and complex flows, while 5 < < 20 and > 20 generally follow accretion onto filaments and galaxy clusters, respectively. By applying results from nonlinear diffusive shock acceleration models using the first-order Fermi process, we calculate the amount of kinetic energy that is converted into cosmic-ray protons. The acceleration of cosmic-ray protons is large enough that in order to use galaxy clusters as cosmological probes, the dynamic response of the gas to the cosmic rays must be included in future numerical simulations.

More reliable constraints on the microlensing optical depth can come from a better understanding of the Galactic model. Based on well-constrained Galactic bulge and disk models constructed from survey observations, such as those from HST, 2MASS, and SDSS, we calculate the microlensing optical depths toward Galactic bulge fields and compare them with recent results from microlensing surveys. We test the χ² statistics of the microlensing optical depths expected from these models, as well as from previously proposed models, using two types of data: optical depth maps in (l, b) and averaged optical depth over Galactic longitude l as a function of latitude b. From this analysis, we find that the Galactic bulge models from 2MASS and Han & Gould, and model G2 of Stanek et al., show good agreement with the microlensing optical depth profiles for all the microlensing observations, compared with model E2 of Stanek et al. On the other hand, we find that models involving an SDSS disk model produce relatively higher χ² values. It should be noted that modeled microlensing optical depths diverge at low Galactic latitudes, |b| ≲ 2°. Therefore, we suggest microlensing observations toward regions much closer to the center of the Galaxy to further test the proposed Galactic models, if technically feasible, rather than waiting for a larger data set of microlensing events.

Due to the high efficiency of planet detections, current microlensing planet searches focus on high-magnification events. High-magnification events are sensitive to remote binary companions as well, and thus a sample of wide-separation binaries are expected to be collected as a by-product. In this paper, we show that characterizing binaries for a portion of this sample will be difficult due to the degeneracy of the binary-lensing parameters. This degeneracy arises because the perturbation induced by the binary companion is well approximated by the Chang-Refsdal lensing for binaries with separations greater than a certain limit. For binaries composed of equal-mass lenses, we find that the lens binarity can be noticed up to separations of ~60 times the Einstein radius corresponding to the mass of each lens. Among these binaries, however, we find that the lensing parameters can be determined only for a portion of binaries with separations less than ~20 times the Einstein radius.

Balmer-dominated shocks in supernova remnants (SNRs) produce strong hydrogen lines with a two-component profile composed of a narrow contribution from cold upstream hydrogen atoms and a broad contribution from hydrogen atoms that have undergone charge transfer reactions with hot protons. Observations of emission lines from edgewise shocks in SNRs can constrain the gas velocity and collisionless electron heating at the shock front. Downstream hydrogen atoms engage in charge transfer, excitation, and ionization reactions, defining an interaction region called the shock transition zone. The properties of hot hydrogen atoms produced by charge transfers (called broad neutrals) are critical for accurately calculating the structure and radiation from the shock transition zone. This paper is the third in a series describing the kinetic, fluid, and emission properties of Balmer-dominated shocks, and it is the first to properly treat the effect of broad neutral kinetics on the shock transition zone structure. We use our models to extract shock parameters from observations of Balmer-dominated SNRs. We find that the inferred shock velocities and electron temperatures are lower than those of previous calculations by <10% for v_s < 1500 km s⁻¹ and by 10%-30% for v_s > 1500 km s⁻¹. This effect is primarily due to the fact that excitation by proton collisions and charge transfer to excited levels favor the high-speed part of the neutral hydrogen velocity distribution. Our results have a strong dependence on the ratio of the electron to proton temperatures, β ≡ T_e/T_p, which allows us to construct a relation β (v_s) between the temperature ratio and the shock velocity. We compare our calculations to previous results by Ghavamian and coworkers.

Rate coefficients for rotational transitions in H₂ induced by H₂ impact are presented. Extensive quantum mechanical coupled-channel calculations based on a recently published (H₂)₂ potential energy surface were performed. The potential energy surface used here has been demonstrated to be more reliable than surfaces used in previous work. Rotational transition cross sections with initial levels of J ⩽ 8 were computed for collision energies ranging between 10⁻⁴ and 2.5 eV, and the corresponding rate coefficients were calculated for the temperature range 2 ⩽ T⩽ 10,000 K. In general, agreement with earlier calculations, which were limited to 100-6000 K, is good, although discrepancies are found at the lowest and highest temperatures. Low-density-limit cooling functions due to para- and ortho-H₂ collisions are obtained from the collisional rate coefficients. Implications of the new results for nonthermal H₂ rotational distributions in molecular regions are also investigated.

We present an analysis of the gas physics at the base of jets from five T Tauri stars based on high angular resolution optical spectra, using the Hubble Space Telescope Imaging Spectrograph (HST STIS). The spectra refer to a region within 100 AU of the star, i.e., where the collimation of the jet has just taken place. We form position-velocity (PV) images of the line ratios to get a global picture of the flow excitation. We then apply a specialized diagnostic technique to find the electron density, ionization fraction, electron temperature, and total density. Our results are in the form of PV maps of the obtained quantities, in which the gas behavior is resolved as a function of both radial velocity and distance from the jet axis. They highlight a number of interesting physical features of the jet collimation region, including regions of extremely high density, asymmetries with respect to the axis, and possible shock signatures. Finally, we estimate the jet mass and angular momentum outflow rates, both of which are fundamental parameters in constraining models of accretion-ejection structures, particularly if the parameters can be determined close to the jet footpoint. Comparing mass flow rates for cases where the mass accretion rate is available in the literature (i.e., for DG Tau, RW Aur, and CW Tau) reveals a mass ejection-to-accretion ratio of 0.01-0.07. Finally, where possible (i.e., for DG Tau and CW Tau), both mass and angular momentum outflow rates are resolved into higher and lower velocity jet material. For the clearer case of DG Tau, this reveals that the more collimated higher velocity component plays a dominant role in mass and angular momentum transport.

We estimate cluster ages from lithium depletion in five pre-main-sequence groups found within 100 pc of the Sun: the TW Hydrae association, η Chamaeleontis cluster, β Pictoris moving group, Tucanae-Horologium association, and AB Doradus moving group. We determine surface gravities, effective temperatures, and lithium abundances for over 900 spectra through least-squares fitting to model-atmosphere spectra. For each group, we compare the dependence of lithium abundance on temperature with isochrones from pre-main-sequence evolutionary tracks to obtain model-dependent ages. We find that the η Cha cluster and the TW Hydrae association are the youngest, with ages of 12 ± 6 Myr and 12 ± 8 Myr, respectively, followed by the β Pic moving group at 21 ± 9 Myr, the Tucanae-Horologium association at 27 ± 11 Myr, and the AB Dor moving group at an age of at least 45 Myr (whereby we can only set a lower limit, since the models—unlike real stars—do not show much lithium depletion beyond this age). Here the ordering is robust, but the precise ages depend on our choice of both atmospheric and evolutionary models. As a result, while our ages are consistent with estimates based on Hertzsprung-Russell isochrone fitting and dynamical expansion, they are not yet more precise. Our observations do show that with improved models, much stronger constraints should be feasible, as the intrinsic uncertainties, as measured from the scatter between measurements from different spectra of the same star, are very low: around 10 K in effective temperature, 0.05 dex in surface gravity, and 0.03 dex in lithium abundance.

We present high angular resolution submillimeter continuum images and molecular line spectra obtained with the Submillimeter Array toward two massive cores that lie within infrared dark clouds (IRDCs), one actively star-forming (G034.43+00.24 MM1) and the other more quiescent (G028.53–00.25 MM1). The high angular resolution submillimeter continuum image of G034.43+00.24 MM1 reveals a compact (~0.03 pc) and massive (~29 M_☉) structure, while the molecular line spectrum shows emission from numerous complex molecules. Such a rich molecular line spectrum from a compact region indicates that G034.43+00.24 MM1 contains a hot molecular core, an early stage in the formation of a high-mass protostar. Moreover, the velocity structure of its ¹³CO (3-2) emission indicates that this B0 protostar may be surrounded by a rotating circumstellar envelope. In contrast, the submillimeter continuum image of G028.53–00.25 MM1 reveals three compact (≲0.06 pc), massive (9-21 M_☉) condensations, but there are no lines detected in its spectrum. We suggest that the core G028.53–00.25 MM1 is in a very early stage in the high-mass star formation process; its size and mass are sufficient to form at least one high-mass star, yet it shows no signs of localized heating. Because the combination of high-velocity line wings with a large IR-to-millimeter bolometric luminosity (~10²L_☉) indicates that this core has already begun to form accreting protostars, we speculate that the condensations may be in the early phase of accretion and may eventually become high-mass protostars. Therefore, we have found the possible existence of two high-mass star-forming cores: one in a very early phase of star formation and one in the later hot-core phase. Together, the properties of these two cores support the idea that the earliest stages of high-mass star formation occur within IRDCs.

We investigate the GeV emission from gamma-ray bursts (GRBs) using the results from the Energetic Gamma Ray Experimental Telescope (EGRET) and in view of the Gamma-Ray Large Area Space Telescope (GLAST). Assuming that the conventional prompt and afterglow photons originate from synchrotron radiation, we compare an accompanying inverse-Compton component with EGRET measurements and upper limits on GeV fluence, taking Klein-Nishina feedback into account. We find that the EGRET constraints are consistent with the theoretical framework of the synchrotron self-Compton model for both prompt and afterglow phases, and discuss constraints on microphysical parameters in both phases. Based on the inverse-Compton model and using EGRET results, we predict that GLAST would detect GRBs with GeV photons at a rate of ≳20 yr⁻¹ from both the prompt and afterglow phases. This rate applies to the high-energy tail of the prompt synchrotron emission and to the inverse-Compton component of the afterglow. Theory predicts that in a large fraction of the cases where synchrotron GeV prompt emission would be detected by GLAST, inverse-Compton photons should also be detected at high energies (≳10 GeV). Therefore, GLAST will enable a more precise test of the high-energy emission mechanism. Finally, we show that the contribution of GRBs to the flux of the extragalactic gamma-ray background measured with EGRET is at least 0.01%, and likely around 0.1%.

Correlation studies of prompt and afterglow emission from gamma-ray bursts (GRBs) between different spectral bands have been difficult to do in the past because few bursts had comprehensive and comparable afterglow measurements. In this paper we present a large and uniform data set for correlation analysis based on bursts detected by the Swift mission. For the first time, short and long bursts can be analyzed and compared. It is found for both classes that the optical, X-ray, and gamma-ray emission are linearly correlated, but with a large spread about the correlation line; stronger bursts tend to have brighter afterglow, and bursts with brighter X-ray afterglow tend to have brighter optical afterglow. Short bursts are, on average, weaker in both prompt and afterglow emission. No short bursts are seen with extremely low optical-to-X-ray ratios, as occurs for "dark" long bursts. Although statistics are still poor for short bursts, there is no evidence yet for a subgroup of short bursts with high extinction, as there is for long bursts. Long bursts are detected in the dark category in the same fraction as pre-Swift bursts. Interesting cases of long bursts that are detected in the optical, and yet have a low enough optical-to-X-ray ratio to be classified as dark, are discovered. For the prompt emission, short and long bursts have different average tracks on flux versus fluence plots. In Swift, GRB detections tend to be fluence-limited for short bursts and flux-limited for long events.

The large range of time and length scales involved in Type Ia supernovae (SNe Ia) requires the use of flame models. As a prelude to exploring various options for flame models, we consider in this paper high-resolution, three-dimensional simulations of the small-scale dynamics of nuclear flames in the supernova environment in which the details of the flame structure are fully resolved. The range of densities examined, (1–8) × 10⁷ g cm⁻³, spans the transition from the laminar flamelet regime to the distributed burning regime where small-scale turbulence disrupts the flame. The use of a low Mach number algorithm facilitates the accurate resolution of the thermal structure of the flame and the inviscid turbulent kinetic energy cascade, while implicitly incorporating kinetic energy dissipation at the grid-scale cutoff. For an assumed background of isotropic Kolmogorov turbulence with an energy characteristic of SNe Ia, we find a transition density between 1 and 3 × 10⁷ g cm⁻³, where the nature of the burning changes qualitatively. By 1 × 10⁷ g cm⁻³, energy diffusion by conduction and radiation is exceeded, on the flame scale, by turbulent advection. As a result, the effective Lewis number approaches unity. That is, the flame resembles a laminar flame but is turbulently broadened with an effective diffusion coefficient, D_T ∼ u^'l, where u^' is the turbulent intensity and l is the integral scale. For the larger integral scales characteristic of a real supernova, the flame structure is predicted to become complex and unsteady. Implications for a possible transition to detonation are discussed.

We report on the discovery of the geometry producing the light echo emanating from supernova 2006X, a nearby but underluminous Type Ia supernova (SN Ia) in M100 (=NGC 4321). This offers a rare chance to study the environment of a SN Ia. Contrary to previous reports, there is little evidence of a circumstellar component in the light echo morphology or in the light curve of the unresolved SN point source. Instead, the obvious and dominant echo contribution comes from what is probably a relatively thin sheet of material some 26 pc in front of SN 2006X. Of the four known SN Ia light echoes, three show no evidence of a circumstellar echo and the fourth needs to be confirmed. We consider other evidence for circumstellar material around SN Ia, which may be rare.

We present an optical spectropolarimetric observation of the Type Ic supernova (SN) 2007gr made with the Subaru Telescope at 21 days after maximum brightness (~37 days after the explosion). Nonzero polarization as high as ~3% is observed at the absorption feature of the Ca II IR triplet. The polarization of the continuum light is ~0.5% if we estimate the interstellar polarization (ISP) component by assuming that the continuum polarization has a single polarization angle. This suggests that the axis ratio of the SN photosphere projected on the sky differs from unity by ~10%. The polarization angle at the Ca II absorption is almost aligned with that of the continuum light. These features may be understood in the context of a model in which a bipolar explosion with an oblate photosphere is being viewed from a slightly off-axis direction and explosively synthesized Ca near the polar region obscures the light that originates around the minor axis of the SN photosphere. Given the uncertainty in the ISP, however, the polarization data could also be interpreted with a model with an almost spherically symmetric photosphere and a clumpy Ca II distribution.

We present simultaneous Chandra High-Energy Transmission Gratings (HETG) and Rossi X-ray Timing Explorer (RXTE) observations of a "soft state" of the black hole candidate 4U 1957+11. These spectra, having limited hard X-ray excess, are an excellent test of disk atmosphere models that include effects of black hole spin. The HETG data show, by modeling the broadband continuum and direct fitting of absorption edges, that the disk spectrum is only very mildly absorbed, with N_H = (1–2) × 10²¹ cm⁻². These data additionally reveal λλ13.449 Ne IX absorption consistent with the warm/hot phase of the interstellar medium. The fitted disk model implies an inclined disk around a low-mass black hole rotating with normalized spin a^* ≈ 1. We show, however, that pure Schwarzschild models describe the data extremely well, albeit with large disk atmosphere ``color-correction'' factors. Standard correction factors can be attained if one incorporates mild Comptonization. We find that the Chandra observations do not uniquely determine spin, even with this otherwise extremely well-measured, nearly pure disk spectrum. XMM-Newton RXTE observations, taken only six weeks later, are equally unconstraining. This lack of constraint is partly driven by the unknown mass and distance of 4U 1957+11; however, it is also driven by the limited Chandra and XMM-Newton bandpasses. We therefore present a series of 48 RXTE observations taken at different brightness/hardness levels. These data prefer a spin of a^* ≈ 1, even when including a mild Comptonization component; however, they also show evolution of the color-correction factors. If the rapid-spin models with standard correction factors are to be believed, then the RXTE observations predict that 4U 1957+11 can range from a 3 M_☉ black hole at 10 kpc with a^* ≈ 0.83 to a 16 M_☉ black hole at 22 kpc with a^* ≈ 1.

We present a detailed analysis of three globular cluster X-ray sources in the XMM-Newton extended survey of M31. The X-ray counterpart to the M31 globular cluster Bo 45 (XBo 45) was observed with XMM-Newton on 2006 December 26. Its combined pn+MOS 0.3-10 keV light curve was seen to vary by ~10%, and its 0.3-7.0 keV emission spectrum was well described by an absorbed power law with photon index 1.44 ± 0.12. Its variability and emission is characteristic of low-mass X-ray binaries (LMXBs) in the low-hard state, whether the accretor is a neutron star or black hole. Such behavior is typically observed at luminosities ≲10% Eddington. However, XBo 45 exhibited this behavior at an unabsorbed, 0.3-10 keV luminosity of 2.5 ± 0.2 × 10³⁸ erg s⁻¹, or ~140% Eddington for a 1.4 M_☉ neutron star accreting hydrogen. Hence, we identify XBo 45 as a new candidate black hole LMXB. XBo 45 appears to have been consistently bright for ~30 years, consistent with theoretical prediction for a globular cluster black hole binary formed via tidal capture. Bo 375 was observed in the 2007 January 2 XMM-Newton observation, and has a two-component spectrum that is typical for a bright neutron star LMXB. Bo 135 was observed in the same field as Bo 45, and could contain either a black hole or a neutron star.

We present X-ray and infrared observations of the X-ray source CXOGC J174536.1–285638. Previous observations suggest that this source may be an accreting binary with a high-mass donor (HMXB) or a colliding wind binary (CWB). Based on the Chandra and XMM-Newton light curve, we have found an apparent 189 ± 6 day periodicity with better than 99.997% confidence. We discuss several possible causes of this periodicity, including both orbital and superorbital interpretations. We explore in detail the possibility that the X-ray modulation is related to an orbital period and discuss the implications for two scenarios; one in which the variability is caused by obscuration of the X-ray source by a stellar wind, and the other in which it is caused by an eclipse of the X-ray source. We find that in the first case, CXOGC J174536.1–285638 is consistent with both CWB and HMXB interpretations, but in the second, CXOGC J174536.1–285638 is more likely a HMXB.

Axisymmetric magnetorotational instability (MRI) in viscous accretion disks is investigated by linear analysis and two-dimensional nonlinear simulations. The linear growth of the viscous MRI is characterized by the Reynolds number defined as R_MRI ≡ v²_A/ν Ω , where v_A is the Alfvén velocity, ν is the kinematic viscosity, and Ω is the angular velocity of the disk. Although the linear growth rate is suppressed considerably as the Reynolds number decreases, the nonlinear behavior is found to be almost independent of R_MRI. At the nonlinear evolutionary stage, a two-channel flow continues growing and the Maxwell stress increases until the end of calculations even though the Reynolds number is much smaller than unity. A large portion of the injected energy to the system is converted to the magnetic energy. The gain rate of the thermal energy, on the other hand, is found to be much larger than the viscous heating rate. Nonlinear behavior of the MRI in the viscous regime and its difference from that in the highly resistive regime can be explained schematically by using the characteristics of the linear dispersion relation. Applying our results to the case with both the viscosity and resistivity, it is anticipated that the critical value of the Lundquist number S_MRI ≡ v²_A/η Ω for active turbulence depends on the magnetic Prandtl number S_{MRI ,c} ∝ Pm^1/2 in the regime of Pm ≫ 1 and remains constant when Pm ≪ 1, where Pm ≡ S_MRI/R_MRI = ν/η and η is the magnetic diffusivity.

Using the Sloan Digital Sky Survey (SDSS) Data Release 6, we construct two independent samples of candidate stellar wide binaries selected (1) as pairs of unresolved sources with angular separation in the range 3''–16'', and (2) as common proper motion pairs with 5''–30'' angular separation, and make them publicly available. These samples are dominated by disk stars, and we use them to constrain the shape of the main-sequence photometric parallax relation M_r(r − i) and to study the properties of wide binary systems. We estimate M_r(r − i) by searching for a relation that minimizes the difference between distance moduli of primary and secondary components of wide binary candidates. We model M_r(r − i) by a fourth degree polynomial and determine the coefficients independently for each sample using Markov chain Monte Carlo fitting. Aided by the derived photometric parallax relation, we construct a series of high-quality catalogs of candidate main-sequence binary stars. Using these catalogs, we study the distribution of semimajor axes of wide binaries, a, in the 2000 AU < a < 47,000 AU range. We find the observations to be well described by the Öpik distribution, f(a) ∝ 1/a, for a < a_break, where a_break increases roughly linearly with the height Z above the Galactic plane [a_break ∝ 12,300 Z(kpc)^0.7 AU]. The number of wide binary systems with 100 AU < a < a_break, as a fraction of the total number of stars, decreases from 0.9% at Z = 0.5 kpc to 0.5% at Z = 3 kpc. The probability for a star to be in a wide binary system is independent of its color. Given this color, the companions of red components seem to be drawn randomly from the stellar luminosity function, while blue components have a larger blue-to-red companion ratio than expected from the luminosity function.

The search for binarity in AGB stars is of critical importance for our understanding of how planetary nebulae acquire the dazzling variety of aspherical shapes which characterizes this class. However, detecting binary companions in such stars has been severely hampered due to their extreme luminosities and pulsations. We have carried out a small imaging survey of AGB stars in ultraviolet light (using GALEX), where these cool objects are very faint, in order to search for hotter companions. We report the discovery of significant far-ultraviolet excesses toward nine of these stars. The far-ultraviolet excess most likely results either directly from the presence of a hot binary companion or indirectly from a hot accretion disk around the companion.

We address the evolution of lithium in Population II dwarf stars under the joint effects of microscopic diffusion and tachocline mixing. This process relies on analytical developments and is also constrained by helioseismology observations. It was successfully applied to solar analogs but never investigated in halo stars. It is induced in the upper radiation zone by rotation and a slight differential rotation in latitude. Consequently we modeled different possible rotation histories of halo stars, showing that the initial rotation rate had no impact on lithium in the framework of tachocline mixing. We find a negligible impact of pre-main-sequence evolution on ⁷Li independent of metallicity provided that [ Fe/H ] < − 1. On the contrary, microscopic diffusion and tachocline turbulence act on the long term of main sequence and shape the current ⁷Li − T_eff pattern from the turnoff down to 5000 K. The tachocline mixing models fit the ⁷Li-T_eff relation better than the pure microscopic diffusion models. We address the issue of warm ⁷Li-poor stars and conclude that a moderate mass transfer from a companion could explain their composition. Finally, we discuss the lithium lighter isotope. The pre-main-sequence and main-sequence ⁶Li depletion we compute seems difficult to reconcile with the current observations.

The sizes and shapes of the stars o Ceti and R Leonis have been measured in the mid-infrared. The observations were made using the UC Berkeley Infrared Spatial Interferometer (ISI), and they reveal details about the size, shape and asymmetry of both stars over several epochs in 2006. The star o Ceti appears to be rather symmetric, while the shape of R Leonis appears more consistent with a uniform disk plus a point source that provides approximately 9% additional intensity somewhere in the southern half of the star.

Using a large sample of optical spectra of late-type dwarfs, we identify a subset of late-M through L field dwarfs that, because of the presence of low-gravity features in their spectra, are believed to be unusually young. From a combined sample of 303 field L dwarfs, we find observationally that 7.6% ± 1.6% are younger than 100 Myr. This percentage is in agreement with theoretical predictions once observing biases are taken into account. We find that these young L dwarfs tend to fall in the southern hemisphere (decl . < 0°) and may be previously unrecognized, low-mass members of nearby, young associations like Tucana-Horologium, TW Hydrae, β Pictoris, and AB Doradus. We use a homogeneously observed sample of ~150 optical spectra to examine lithium strength as a function of L/T spectral type and further corroborate the trends noted by Kirkpatrick and coworkers. We use our low-gravity spectra to investigate lithium strength as a function of age. The data weakly suggest that for early- to mid-L dwarfs the line strength reaches a maximum for a few × 100 Myr, whereas for much older (few Gyr) and much younger (<100 Myr) L dwarfs the line is weaker or undetectable. We show that a weakening of lithium at lower gravities is predicted by model atmosphere calculations, an effect partially corroborated by existing observational data. Larger samples containing L dwarfs of well-determined ages are needed to further test this empirically. If verified, this result would reinforce the caveat first cited by Kirkpatrick and coworkers that the lithium test should be used with caution when attempting to confirm the substellar nature of the youngest brown dwarfs.

We present new evolution sequences for very low mass stars, brown dwarfs, and giant planets and use them to explore a variety of influences on the evolution of these objects. While the predicted adiabatic evolution of luminosity with time is very similar to results of previous work, the remaining disagreements reveal the magnitude of current uncertainty in brown dwarf evolution theory. We discuss the sources of those differences and argue for the importance of the surface boundary condition provided by atmosphere models including clouds. The L- to T-type ultracool dwarf transition can be accommodated within the Ackerman and Marley cloud model by varying the cloud sedimentation parameter. We develop a simple model for the evolution across the L/T transition. By combining the evolution calculation and our atmosphere models, we generate colors and magnitudes of synthetic populations of ultracool dwarfs in the field and in Galactic clusters. We focus on near-infrared color-magnitude diagrams (CMDs) and on the nature of the "second parameter" that is responsible for the scatter of colors along the T_eff sequence. Instead of a single second parameter we find that variations in metallicity and cloud parameters, unresolved binaries, and possibly a relatively young population all play a role in defining the spread of brown dwarfs along the cooling sequence. We also find that the transition from cloudy L dwarfs to cloudless T dwarfs slows down the evolution and causes a pileup of substellar objects in the transition region, in contradiction with previous studies. However, the same model is applied to the Pleiades brown dwarf sequence with less success. Taken at face value, the present Pleiades data suggest that the L/T transition occurs at lower T_eff for lower gravity objects, such as those found in young Galactic clusters. The simulated populations of brown dwarfs also reveal that the phase of deuterium burning produces a distinctive feature in CMDs that should be detectable in ~50-100 Myr old clusters.

Measuring the albedo of an extrasolar planet provides insight into its atmospheric composition and its global thermal properties, including heat dissipation and weather patterns. Such a measurement requires very precise photometry of a transiting system, fully sampling many phases of the secondary eclipse. Space-based optical photometry of the transiting system HD 209458 from the MOST (Microvariablity and Oscillations of Stars) satellite, spanning 14 and 44 days in 2004 and 2005, respectively, allows us to set a sensitive limit on the optical eclipse of the hot exosolar giant planet in this system. Our best fit to the observations yields a flux ratio of the planet and star of 7 ± 9 ppm (parts per million), which corresponds to a geometric albedo through the MOST bandpass (400-700 nm) of A_g = 0.038 ± 0.045. This gives a 1 σ upper limit of 0.08 for the geometric albedo and a 3 σ upper limit of 0.17. HD 209458b is significantly less reflective than Jupiter (for which A_g would be about 0.5). This low geometric albedo rules out the presence of bright reflective clouds in this exoplanet's atmosphere. We determine refined parameters for the star and exoplanet in the HD 209458 system based on a model fit to the MOST light curve.

In the solar convection zone, rotation couples with intensely turbulent convection to drive a strong differential rotation and achieve complex magnetic dynamo action. Our Sun must have rotated more rapidly in its past, as is suggested by observations of many rapidly rotating young solar-type stars. Here we explore the effects of more rapid rotation on the global-scale patterns of convection in such stars and the flows of differential rotation and meridional circulation, which are self-consistently established. The convection in these systems is richly time-dependent, and in our most rapidly rotating suns a striking pattern of localized convection emerges. Convection near the equator in these systems is dominated by one or two nests in longitude of locally enhanced convection, with quiescent streaming flow in between them at the highest rotation rates. These active nests of convection maintain a strong differential rotation despite their small size. The structure of differential rotation is similar in all of our more rapidly rotating suns, with fast equators and slower poles. We find that the total shear in differential rotation Δ Ω grows with more rapid rotation, while the relative shear Δ Ω/Ω₀ decreases. In contrast, at more rapid rotation, the meridional circulations decrease in energy and peak velocities and break into multiple cells of circulation in both radius and latitude.

Far-side imaging using time-distance helioseismology methods is assessed using numerically generated artificial data. The data are generated using direct numerical simulations of acoustic oscillations in a spherical solar model. Localized variations of the sound speed in the surface and subsurface layers are used to model the perturbations associated with sunspots and active regions. The accuracy of acoustic travel-time far-side maps is shown to depend on the size and location of active regions. Potential artifacts in the far-side imaging procedure, such as those caused by the presence of active regions on the solar near side, are also investigated.

In order to understand the influence of magnetic fields on the propagation properties of waves, as derived from different local helioseismology techniques, forward modeling of waves is required. Such calculations need a model in magnetohydrostatic equilibrium as an initial atmosphere through which to propagate oscillations. We provide a method to construct such a model in equilibrium for a wide range of parameters, for use in simulations of artificial helioseismologic data. The method combines the advantages of self-similar solutions and current-distributed models. A set of models is developed by numerical integration of magnetohydrostatic equations from the subphotospheric to chromospheric layers.

In Paper I, we introduced and tested a method for predicting solar active region coronal emissions using magnetic field measurements and a chosen heating relationship. Here, we apply this forward-modeling technique to 10 active regions observed with the Mees Solar Observatory Imaging Vector Magnetograph and the Yohkoh Soft X-ray Telescope. We produce synthetic images of each region using four parameterized heating relationships depending on magnetic field strength and geometry. We find a volumetric coronal heating rate (dE_H/dV, not to be confused with dE_H/dA quoted by some authors) proportional to magnetic field and inversely proportional to field-line loop length (BL⁻¹) best matches observed coronal emission morphologies. This parameterization is most similar to the steady-state scaling of two proposed heating mechanisms: van Ballegooijen's "current layers" theory, taken in the AC limit, and Parker's "critical angle" mechanism, in the case where the angle of misalignment is a twist angle. Although this parameterization best matches the observations, it does not match well enough to make a definitive statement as to the nature of coronal heating. Instead, we conclude that (1) the technique requires better magnetic field measurement and extrapolation techniques than currently available, and (2) forward-modeling methods that incorporate properties of transiently heated loops are necessary to make a more conclusive statement about coronal heating mechanisms.

The physical modeling of active regions (ARs) and of the global corona is receiving increasing interest lately. Recent attempts to model ARs using static equilibrium models were quite successful in reproducing AR images of hot soft X-ray (SXR) loops. They however failed to predict the bright extreme-ultraviolet (EUV) warm loops permeating ARs: the synthetic images were dominated by intense footpoint emission. We demonstrate that this failure is due to the very weak dependence of loop temperature on loop length which cannot simultaneously account for both hot and warm loops in the same AR. We then consider time-dependent AR models based on nanoflare heating. We demonstrate that such models can simultaneously reproduce EUV and SXR loops in ARs. Moreover, they predict radial intensity variations consistent with the localized core and extended emissions in SXR and EUV AR observations, respectively. We finally show how the AR morphology can be used as a gauge of the properties (duration, energy, spatial dependence, and repetition time) of the impulsive heating.

In this paper, we devote ourselves to interpreting the short-lived absorptive type III-like microwave bursts in the 2006 December 13 flare event observed with high temporal and spectral resolutions (8 ms and 10 MHz) by the Chinese Solar Broadband Radio Spectrometer (SBRS/Huairou) at 2.6-3.8 GHz. In the decimeter-centimeter wavelength range, we first present the observations of short-lived bursts represented as a number of absorptive "spikes" superposed on the type IV continuum that can be connected by fast-drifting lines. The mean drift rate, the instantaneous bandwidth, and the absorption depth of these absorptive spikes are about –12 GHz s⁻¹, 70 MHz, and 40%, respectively. The duration at a single frequency band can be less than the instrument resolution of 8 ms. On the basis of numerical investigations of the loss-cone instability, we suggest that fragmented electron injections with durations of as short as several milliseconds into the loss cone could be the most appropriate mechanism with which to explain the bursts. The length of an electron beam is estimated to be about 400 km, on the basis of the observational results. These injections may be related to the fragmented energy release processes during the flare. We also observe some absorptive type III-like bursts accompanying ordinary type III bursts with reverse drifts. They start at the same frequency, and the starting frequency slowly drifts to the low-frequency region. This could be a signature of propagating bidirectional electron beams originating near the reconnection region.

We quantitatively investigate intensity fluctuations observed with the Yohkoh SXT, which is sensitive to hot (>2 MK) plasma, and TRACE, which is sensitive to cool (~1 MK) plasma. We find that the TRACE light curves contain fluctuations that are significantly larger than the photon noise and that TRACE is more sensitive to the emission from nanoflare heating than is the SXT. We discover that the standard deviation of the fluctuation (the photon noise is removed) is proportional to the mean intensity for both the SXT and TRACE loops. We also analyze the autocorrelation functions in order to obtain the duration of the intensity fluctuations. While the duration of the intensity fluctuations for the SXT loops is relatively short because of the significant photon noise, that for the TRACE loops agrees well with the characteristic cooling timescale. This is evidence that coronal loops are continuously heated by impulsive nanoflares. We estimate the energy of nanoflares to be 10²⁵ ergs for SXT loops and 10²³ ergs for TRACE loops. The occurrence rate of nanoflares is about 0.4 and 30 nanoflares s⁻¹ in a typical SXT loop and a typical TRACE loop, respectively.

For investigating the magnetic causes of coronal mass ejections (CMEs) and for forecasting the CME productivity of active regions, in previous work we have gauged the total nonpotentiality of a whole active region by either of two measures, L_SSM and L_SGM, two measures of the magnetic field along the main neutral line in a vector magnetogram of the active region. This previous work was therefore restricted to nominally bipolar active regions, active regions that have a clearly identifiable main neutral line. In the present paper, we show that our work can be extended to include multipolar active regions of any degree of magnetic complexity by replacing L_SSM and L_SGM with their generalized counterparts, WL_SS and WL_SG, which are corresponding integral measures covering all neutral lines in an active region instead of only the main neutral line. In addition, we show that for active regions within 30 heliocentric degrees of disk center, WL_SG can be adequately measured from line-of-sight magnetograms instead of vector magnetograms. This approximate measure of active-region total nonpotentiality,^LWL_SG, with the extensive set of 96 minute cadence full-disk line-of-sight magnetograms from SOHO MDI, can be used to study the evolution of active-region total nonpotentiality leading to the production of CMEs.

Ulysses, launched in 1990 October in the maximum phase of solar cycle 22, completed its third out-of-ecliptic orbit in 2008 February. This provides a unique opportunity to study the propagation of cosmic rays over a wide range of heliographic latitudes during different levels of solar activity and different polarities in the inner heliosphere. Comparison of the first and second fast latitude scans from 1994 to 1995 and from 2000 to 2001 confirmed the expectation of positive latitudinal gradients at solar minimum versus an isotropic Galactic cosmic ray distribution at solar maximum. During the second scan in mid-2000, the solar magnetic field reversed its global polarity. From 2007 to 2008, Ulysses made its third fast latitude scan during the declining phase of solar cycle 23. Therefore, the solar activity is comparable in 2007-2008 to that from 1994 to 1995, but the magnetic polarity is opposite. Thus, one would expect to compare positive with negative latitudinal gradients during these two periods for protons and electrons, respectively. In contrast, our analysis of data from the Kiel Electron Telescope aboard Ulysses results in no significant latitudinal gradients for protons. However, the electrons show, as expected, a positive latitudinal gradient of ~0.2% per degree. Although our result is surprising, the nearly isotropic distribution of protons in 2007-2008 is consistent with an isotropic distribution of electrons from 1994 to 1995.

In light of recent measurement of nitrogen isotope ratios in CN and HCN in several comets, and the correlation between ¹⁵N excess and the presence of nitrile (-CN) functional groups in meteoritic samples, we have reassessed the potential of interstellar chemistry to directly fractionate nitriles. We focus in particular on the ¹⁵N chemistry in selective depletion cores where O-bearing molecules are depleted yet N- and C-bearing species remain in the gas, as revealed by the recent detection of CN in dense CO-depleted cores. We show that large HC¹⁵N/HC¹⁴N ratios can be generated if the reaction of CN with N has a barrier, and suggest that cometary HCN and CN may trace material originally formed in dense interstellar clouds.

We study the impact of inhomogeneous hydrogen reionization on the thermal evolution of the intergalactic medium (IGM) using hydrodynamic + radiative transfer simulations where reionization is completed either early (z ∼ 9) or late (z ∼ 6). In general, we find that low-density gas near large-scale overdensities is ionized and heated earlier than gas in the large-scale, underdense voids. Furthermore, at a later time the IGM temperature is inversely related to the reionization redshift because gas that is heated earlier has more time to cool through adiabatic expansion and Compton scattering. Thus, at the end of reionization the median temperature-density relation is an inverted power law with slope γ − 1 ∼ − 0.2, in both models. However, at fixed density there is up to order unity scatter in the temperature due to the distribution of reionization redshifts. Because of the complex equation of state, the evolved IGM temperature-density relations for the redshift range 4 ≲ z ≲ 6 can still have significant curvature and scatter. These features must be taken into account when interpreting the Lyα absorption in high-redshift quasar spectra.

A quantitative theory of spectral lags for γ-ray bursts (GRBs) is given. The underlying hypothesis is that GRB subpulses are photons that are scattered into our line of sight by local concentrations of baryons that are accelerated by radiation pressure. For primary spectra that are power laws with exponential cutoffs, the width of the pulse and its fast rise, slow decay asymmetry is found to increase with decreasing photon energy, and the width near the exponential cutoff scales approximately as E_ph^−η, where η ∼ 0.4, as observed. The spectral lag time is naturally inversely proportional to luminosity, all else being equal, also as observed.

We consider the consequences of gravitational wave recoil for unified models of active galactic nuclei (AGNs). Spatial oscillations of supermassive black holes (SMBHs) around the cores of galaxies following gravitational wave (GW) recoil imply that the SMBHs spend a significant fraction of time off-nucleus, at scales beyond that of the molecular obscuring torus. Assuming reasonable distributions of recoil velocities, we compute the off-core timescale of (intrinsically type 2) quasars. We find that roughly one-half of major mergers result in a SMBH being displaced beyond the torus for a time of 10^7.5 yr or more, comparable to quasar activity timescales. Since major mergers are most strongly affected by GW recoil, our results imply a deficiency of type 2 quasars in comparison to Seyfert 2 galaxies. Other consequences of the recoil oscillations for the observable properties of AGNs are also discussed.

We combine the recent estimate of the contribution of galaxies to the 3.6 μm intensity of the extragalactic background light (EBL) with optical and near-infrared (IR) galaxy counts to set new limits on intrinsic spectra of some of the most distant TeV blazars, 1ES 0229+200, 1ES 1218+30.4, and 1ES 1101–232, located at redshifts 0.1396, 0.182, and 0.186, respectively. The new lower limit on the 3.6 μm EBL intensity is significantly higher than the previous one set by the cumulative emission from resolved Spitzer galaxies. Correcting for attenuation by the revised EBL, we show that the differential spectral index of the intrinsic spectrum of the three blazars is 1.28 ± 0.20 or harder. These results present blazar emission models with the challenge of producing extremely hard intrinsic spectra in the sub-TeV to multi-TeV regime. These results also question the reliability of recently derived upper limits on the near-IR EBL intensity that are solely based on the assumption that intrinsic blazar spectra should not be harder than 1.50.

We report the discovery of a confirmed supernova (SN) and a supernova candidate in near-infrared images from the ALTAIR/NIRI adaptive optics system on the Gemini-North Telescope and NICMOS on the Hubble Space Telescope. The Gemini images were obtained as part of a near-infrared K-band search for highly obscured SNe in the nuclear regions of luminous infrared galaxies. SN 2008cs, apparent in the Gemini images, is the first SN discovered using laser guide star adaptive optics. It is located at 1500 pc projected distance from the nucleus of the luminous infrared galaxy IRAS 17138–1017. The SN luminosity, JHK colors, and light curve are consistent with a core-collapse event suffering from a very high host galaxy extinction of 15.7 ± 0.8 mag in the V-band, which is to our knowledge the highest yet measured for a SN. The core-collapse nature of SN 2008cs is confirmed by its radio detection at 22.4 GHz using our Very Large Array observations 28 days after the SN discovery, indicating a prominent interaction of the SN ejecta with the circumstellar medium. An unconfirmed SN apparent in the NICMOS images from 2004 is located in the same galaxy at 660 pc projected distance from the nucleus and has a lower extinction.

Observations have shown that passively evolving massive galaxies at high redshift are much more compact than local galaxies with the same stellar mass. We argue that the observed strong evolution in size is directly related to the quasar feedback, which removes huge amounts of cold gas from the central regions in a Salpeter time, inducing an expansion of the stellar distribution. The new equilibrium configuration, with a size increased by a factor ≳3, is attained after ~40 dynamical times, corresponding to ~2 Gyr. This means that massive galaxies observed at z ⩾ 1 will settle on the fundamental plane by z ∼ 0.8-1. In less massive galaxies (M_⋆≲ 2 × 10¹⁰M_☉), the nuclear feedback is subdominant, and the mass loss is mainly due to stellar winds. In this case, the mass-loss timescale is longer than the dynamical time and results in adiabatic expansion that may increase the effective radius by a factor of up to ~2 in 10 Gyr, although a growth by a factor of ≃1.6 occurs within the first 0.5 Gyr. Since observations are focused on relatively old galaxies, with ages ≳1 Gyr, the evolution for smaller galaxies is more difficult to perceive. Significant evolution of velocity dispersion is predicted for both small and large galaxies.

Accretion and merger shocks in clusters of galaxies are potential accelerators of high-energy protons, which can give rise to high-energy neutrinos through pp interactions with the intracluster gas. We discuss the possibility that protons from cluster shocks make a significant contribution to the observed cosmic rays in the energy range between the second knee at ~10^17.5 eV and the ankle at ~10^18.5 eV. The accompanying cumulative neutrino background above ~PeV may be detectable by upcoming neutrino telescopes such as IceCube or KM3NeT, providing a test of this scenario as well as a probe of cosmic-ray confinement properties in clusters.

We report the detection of ¹³CO J = 6→ 5 emission from the nucleus of the starburst galaxy NGC 253 with the redshift (z) and Early Universe Spectrometer (ZEUS), a new submillimeter grating spectrometer. This is the first extragalactic detection of the ¹³CO J = 6→ 5 transition, which traces warm, dense molecular gas. We employ a multiline LVG analysis and find ≈35%-60% of the molecular interstellar medium is both warm (T ∼ 110 K) and dense (n_H2 ∼ 10⁴ cm⁻³). We analyze the potential heat sources and conclude that ultraviolet and X-ray photons are unlikely to be energetically important. Instead, the molecular gas is most likely heated by an elevated density of cosmic rays or by the decay of supersonic turbulence through shocks. If the cosmic rays and turbulence are created by stellar feedback within the starburst, then our analysis suggests the starburst may be self-limiting.

We present medium-resolution spectra of 16 radial velocity red-giant members of the low-luminosity Boötes I dwarf spheroidal (dSph) galaxy that have sufficient S/N for abundance determination, based on the strength of the Ca II K line. Assuming [Ca/Fe] ~ 0.3, the abundance range in the sample is Δ[Fe/H] ~ 1.7 dex, with one star having [Fe/H] = –3.4. The dispersion is σ([Fe/H]) = 0.45 ± 0.08—similar to those of the Galaxy's more luminous dSph systems and ω Centauri. This suggests that the large mass (≳10⁷M_☉) normally assumed to foster self-enrichment and the production of chemical abundance spreads was provided by the nonbaryonic material in Boötes I.

We have obtained radial velocity measurements for stars in two widely separated fields in the Anticenter Stream. Combined with SDSS/USNO-B proper motions, the new measurements allow us to establish that the stream is on a nearly circular, somewhat inclined, prograde orbit around the Galaxy. While the orbital eccentricity is similar to that previously determined for the Monoceros stream, the sizes, inclinations, and positions of the orbits for the two systems differ significantly. Integrating our best-fitting Anticenter Stream orbit forward, we find that it is closely aligned along and lies almost on top of a streamlike feature previously designated the "Eastern Banded Structure." The position of this feature coincides with the apogalacticon of the orbit. We tentatively conclude that this feature is the next wrap of the Anticenter Stream.

Vela X is a pulsar wind nebula (PWN) associated with the active pulsar B0833–45 and contained within the Vela supernova remnant (SNR). A collimated X-ray filament ("cocoon") extends south-southwest from the pulsar to the center of Vela X. VLA observations uncovered radio emission coincident with the eastern edge of the cocoon, and H.E.S.S. has detected TeV γ-ray emission from this region as well. Using XMM-Newton archival data, covering the southern portion of this feature, we analyze the X-ray properties of the cocoon. The X-ray data are best fit by an absorbed nonequilibrium plasma model with a power-law component. Our analysis of the thermal emission shows enhanced abundances of O, Ne, and Mg within the cocoon, indicating the presence of ejecta-rich material from the propagation of the SNR reverse shock, consistent with Vela X being a disrupted PWN. We investigate the physical processes that excite the electrons in the PWN to emit in the radio, X-ray, and γ-ray bands. The radio and nonthermal X-ray emission can be explained by synchrotron emission. We model the γ-ray emission by inverse Compton scattering of electrons off of cosmic microwave background (CMB) photons. We use a three-component broken power law to model the synchrotron emission, finding an intrinsic break in the electron spectrum at ~5 × 10⁶ keV and a cooling break at ~5.5 × 10¹⁰ keV. This cooling break along with a magnetic field strength of 5 × 10⁻⁶ G indicate that the synchrotron break occurs at ~1 keV.

Morphologically, it appears as if the Vela X pulsar wind nebula (PWN) consists of two emission regions: whereas X-ray (~1 keV) and very high energy (VHE) H.E.S.S. γ-ray observations appear to define a cocoon-type shape south of the pulsar, radio observations reveal an extended area of size 2° × 3° (including the cocoon area), also south of the Vela pulsar. Since no wide field of view (FoV) observations of the synchrotron emission between radio and X-rays are available, we do not know how the lepton (e^±) spectra of these two components connect or how the morphology changes with energy. Currently, we find that two distinct lepton spectra describe the respective radio and X-ray/VHE γ-ray spectra, with a field strength of 5 μG self-consistently describing a radiation spectral break (or energy maximum) in the multi-TeV domain as observed by H.E.S.S. (if interpreted as IC radiation), while predicting the total hard X-ray flux above 20 keV (measured by the wide FoV INTEGRAL instrument) within a factor of 2. If this same field strength is also representative of the radio structure (including filaments), the implied IC component corresponding to the highest radio frequencies should reveal a relatively bright high-energy γ-ray structure, and Fermi LAT should be able to resolve it. A higher field strength in the filaments would, however, imply fewer leptons in Vela X and hence a fainter Fermi LAT signal.

We present X-ray observations of the transient accretion-powered millisecond pulsar IGR J00291+5934 during quiescence. IGR J00291+5934 is the first source among accretion-powered millisecond pulsars to show signs of a thermal component in its quiescent spectrum. Fitting this component with a neutron star atmosphere or a blackbody model we obtain soft temperatures (~64 and ~110 eV, respectively). As in other sources of this class a hard spectral component is also present, comprising more than 60% of the unabsorbed 0.5-10 keV flux. Interpreting the soft component as cooling emission from the neutron star, we can conclude that the compact object can be spun up to millisecond periods by accreting only ≲0.2 M_☉.

Nonthermal X-ray emission in some supernova remnants originates from synchrotron radiation of ultrarelativistic particles in turbulent magnetic fields. We address the effect of a random magnetic field on synchrotron emission images and spectra. A random magnetic field is simulated to construct synchrotron emission maps of a source with a steady distribution of ultrarelativistic electrons. Nonsteady localized structures (dots, clumps, and filaments), in which the magnetic field reaches exceptionally high values, typically arise in the random field sample. These magnetic field concentrations dominate the synchrotron emission (integrated along the line of sight) from the highest energy electrons in the cutoff regime of the distribution, resulting in an evolving, intermittent, clumpy appearance. The simulated structures resemble those observed in X-ray images of some young supernova remnants. The lifetime of X-ray clumps can be short enough to be consistent with that observed even in the case of a steady particle distribution. The efficiency of synchrotron radiation from the cutoff regime in the electron spectrum is strongly enhanced in a turbulent field compared to emission from a uniform field of the same magnitude.

We report new and archival K-band interferometric uniform disk diameters obtained with the Palomar Testbed Interferometer for the eclipsing binary star Aurigae, in advance of the start of its eclipse in 2009. The observations were intended to test whether low-amplitude variations in the system are connected with the F supergiant star (primary), or with the intersystem material connecting the star with the enormous dark disk (secondary) inferred to cause the eclipses. Cepheid-like radial pulsations of the F star are not detected, nor do we find evidence for proposed 6% per decade shrinkage of the F star. The measured 2.27 ± 0.11 mas K-band diameter is consistent with a 300 solar radius F supergiant star at the Hipparcos distance of 625 pc. These results provide an improved context for observations during the 2009-2011 eclipse.

Studying the earliest stages in the birth of stars is crucial for understanding how they form. Brown dwarfs with masses between that of stars and planets are not massive enough to maintain stable hydrogen-burning fusion reactions during most of their lifetime. Their origins are subject to much debate in recent literature because their masses are far below the typical mass where core collapse is expected to occur. We present the first confirmed evidence that brown dwarfs undergo a phase of molecular outflow that is typical of young stars. Using the Submillimeter Array, we have obtained a map of a bipolar molecular outflow from a young brown dwarf. We estimate an outflow mass of 1.6 × 10⁻⁴M_☉ and a mass-loss rate of 1.4 × 10⁻⁹M_☉. These values are over 2 orders of magnitude smaller than the typical ones for T Tauri stars. From our millimeter continuum data and our own analysis of Spitzer infrared photometry, we estimate that the brown dwarf has a disk with a mass of 8 × 10⁻³M_☉ and an outer disk radius of 80 AU. Our results demonstrate that the bipolar molecular outflow operates down to planetary masses, occurring in brown dwarfs as a scaled-down version of the universal process seen in young stars.

Here we present the Spitzer IRS spectrum of CVSO 224, the sole transitional disk located within the ~10 Myr old 25 Orionis group in Orion OB1a. A model fit to the spectral energy distribution of this object indicates a ~7 AU inner disk hole that contains a small amount of optically thin dust. In previous studies, CVSO 224 had been classified as a weak-line T Tauri star based on its Hα equivalent width, but here we find an accretion rate of 7 × 10⁻¹¹M_☉ yr⁻¹ based on high-resolution Hectochelle data. CVSO 224's low is in line with photoevaporative clearing theories. However, the Spitzer IRS spectrum of CVSO 224 has a substantial mid-infrared excess beyond 20 μm which indicates that it is surrounded by a massive outer disk. Millimeter measurements are necessary to constrain the mass of the outer disk around CVSO 224 in order to confirm that photoevaporation is not the mechanism behind creating its inner disk hole.

We present ground-based observations of the transiting Neptune-mass planet Gl 436b obtained with the 3.5 m telescope at Apache Point Observatory and other supporting telescopes. Included in this is an observed transit in early 2005, over 2 years before the earliest reported transit detection. We have compiled all available transit data to date and perform a uniform modeling using the JKTEBOP code. We do not detect any transit timing variations of amplitude greater than ~1 minute over the ~3.3 year baseline. We do however find possible evidence for a self-consistent trend of increasing orbital inclination, transit width, and transit depth, which supports the supposition that Gl 436b is being perturbed by another planet of ≲12 M_⊕ in a nonresonant orbit.

We present Gemini near-infrared adaptive optics imaging and spectroscopy of a planetary-mass candidate companion to 1RXS J160929.1–210524, a roughly solar-mass member of the 5 Myr old Upper Scorpius association. The object, separated by 2.22'' or 330 AU at ~150 pc, has infrared colors and spectra suggesting a temperature of 1800₋₁₀₀⁺²⁰⁰ K, and spectral type of L4₋₂⁺¹. The H- and K-band spectra provide clear evidence of low surface gravity, and thus youth. Based on the widely used DUSTY models, we infer a mass of 8₋₂⁺⁴M_Jup. If gravitationally bound, this would be the lowest mass companion imaged around a normal star thus far, and its existence at such a large separation would pose a serious challenge to theories of star and planet formation.

The possible role of magnetic flux emergence in the initiation of coronal mass ejections (CMEs) is investigated in the framework of the breakout model. The ideal MHD equations are solved numerically on a spherical, axisymmetric (2.5-dimensional) domain. An initial multiflux system in steady equilibrium containing a pre-eruptive region consisting of three arcades with alternating magnetic flux polarity is kept in place by the magnetic tension of the overlying closed magnetic field of a helmet streamer. The emergence of new magnetic flux in the central arcade is simulated by means of a time-dependent boundary condition on the vector potential applied at the solar base. Height-time plots of the ejected material, as well as time evolution of the magnetic, kinetic and internal energy in the entire domain as functions of flux emergence rate, are produced. The results show that the emergence of new magnetic flux in the central arcade triggers a CME. The obtained eruption corresponds to a slow CME, and conversion of magnetic energy into kinetic energy is observed.

We perform helioseismic holography to assess the noise in p-mode travel-time shifts which would form the basis of inferences of large-scale flows throughout the solar convection zone. We also derive the expected travel times from a parameterized return (equatorward) flow component of the meridional circulation at the base of the convection zone from forward models under the assumptions of the ray and Born approximations. From estimates of the signal-to-noise ratio for measurements focused near the base of the convection zone, we conclude that the helioseismic detection of the deep meridional flow including the return component may not be possible using travel-time measurements spanning an interval less than a solar cycle. We speculate that this conclusion may be generally true for other helioseismic methods under the assumption that the underlying measurements are equivalently limited by solar realization noise.