Table of contents

The first galaxies in the universe are built up where cold dark matter (CDM) forms large-scale filamentary structure. Although the galaxies are expected to emit numerous Lyα photons, they are surrounded by plentiful neutral hydrogen with a typical optical depth for Lyα of ~10⁵ (H I halos) before the era of cosmological reionization. The H I halo almost follows the cosmological Hubble expansion with some anisotropic corrections around the galaxy because of gravitational attraction by the underlying CDM filament. In this paper, we investigate the detectability of the Lyα emissions from the first galaxies, examining their dependence on viewing angles. Solving the Lyα line transfer problem in an anisotropically expanding H I halo, we show that the escape probability from the H I halo is the largest in the direction along the filament axis. If the Lyα source is observed with a narrowband filter, the difference in apparent Lyα line luminosities among viewing angles can be a factor of ≳40 for an extreme case. Furthermore, we evaluate the predicted physical features of the Lyα sources and flux magnification by the gravitational lensing effect due to clusters of galaxies along the filament. We conclude that by using next-generation space telescopes such as JWST, the Lyα emissions from the first galaxies whose CDM filament axes almost face us can be detected with S/N ≳ 10.

In the brane world scenario, the four-dimensional effective Einstein equation has extra terms that arise from the embedding of the 3-brane in the bulk. These nonlocal effects may provide an explanation for the dynamics of the neutral hydrogen clouds at large distances from the galactic center, which is usually explained by postulating the existence of the dark matter. We obtain the exact galactic metric, the dark radiation, and the dark pressure in the flat rotation curve region in the brane world scenario. Due to the presence of the bulk effects, the flat rotation curves could extend several hundred kiloparsecs. The limiting radius for which bulk effects are important is compared with the numerical values of the truncation parameter of the dark matter halos, obtained from weak-lensing observations. There is a relatively good agreement between the predictions of the model and observations. The deflection of photons in the flat rotation curve region is also considered. The bending angle predicted by the brane world models is much larger than that predicted by dark matter models. The angular radii of the Einstein rings are obtained in the small-angle approximation. The tangential shear is compared with the observational data obtained in the weak lensing of galaxies in the Red-Sequence Cluster Survey. The study of the light deflection by galaxies and the gravitational lensing could discriminate between the different dynamical laws proposed to model the motion of particles at the galactic level and the standard dark matter scenario.

We perform a systematic analysis of the effects of photometric redshift uncertainties on weak-lensing tomography. We describe the photo-z distribution with a bias and Gaussian scatter that are allowed to vary arbitrarily between intervals of δz = 0.1 in redshift. While the mere presence of bias and scatter does not substantially degrade dark energy information, uncertainties in both parameters do. For a fiducial next-generation survey each would need to be known to better than about 0.003-0.01 in redshift for each interval in order to lead to less than a factor of 1.5 increase in the dark energy parameter errors. The more stringent requirement corresponds to a larger dark energy parameter space, when redshift variation in the equation of state of dark energy is allowed. Of order 10⁴-10⁵ galaxies with spectroscopic redshifts fairly sampled from the source galaxy distribution will be needed to achieve this level of calibration. If the sample is composed of multiple galaxy types, a fair sample would be required for each. These requirements increase in stringency for more ambitious surveys; we quantify such scalings with a convenient fitting formula. No single aspect of a photometrically binned selection of galaxies such as their mean or median suffices, indicating that dark energy parameter determinations are sensitive to the shape and nature of outliers in the photo-z redshift distribution.

We have carried out a deep narrowband imaging survey of six fields with heavy-element quasar absorption lines, using the Goddard Fabry-Pérot (FP) system at the Apache Point Observatory (APO) 3.5 m telescope. The aim of these observations was to search for redshifted Lyα emission from the galaxies underlying the absorbers at z = 2.3-2.5 and their companion galaxies. The 3 σ sensitivity levels ranged between 1.9 × 10^-17 and 5.4 × 10^-17 ergs s^-1 cm^-2 in observed-frame Lyα flux. No significant Lyα emitters were detected at a level >3 σ. The absence of significant Lyα emission implies limits on the star formation rate (SFR) of 0.9-2.7 M_☉ yr^-1 per 2 pixel × 2 pixel region, if no dust attenuation is assumed. We compare our results with those from other emission-line studies of absorber fields and with predictions for the global average SFR based on the models of cosmic chemical evolution. Our limits are among the tightest existing constraints on Lyα emission from galaxies in absorber fields, but they are consistent with many other studies. In the absence of dust attenuation, these studies suggest that SFRs in a large fraction of objects in the absorber fields may lie below the global mean SFR. However, it is possible that dust attenuation is responsible for the low emission-line fluxes in some objects. It is also possible that the star-forming regions are compact and at smaller angular separations from the quasar than the width of our point-spread function and get lost in the quasar emission. We outline future observations that could help to distinguish between the various possibilities.

The effectiveness of the thermal coupling of ions and electrons in optically thin, hot accretion flows is investigated in a phenomenological approach. In the limit of complete coupling, we focus on the one-temperature accretion flows around black holes. Based on a global analysis, the results are compared with two-temperature accretion flow models and with the observations of black hole sources. Many features of one- and two-temperature solutions are quite similar. That is, hot one-temperature solutions are found to exist for mass flow rates less than a critical value, i.e., ≲ 10α²_Edd, where _Edd = L_Edd/c² is the Eddington accretion rate. When ≲ 10^-3α²_Edd, the viscous energy is mainly balanced by the advective cooling, i.e., the solution is in the advection-dominated accretion flow (ADAF) regime. On the other hand, when 10^-3α²_Edd ≲ ≲ 10α²_Edd, radiative cooling is effective and is mainly balanced by advective heating, placing the solution in the regime of luminous hot accretion flow (LHAF). When ≳ 10α²_Edd, the accretion flow collapses at a transition radius with only the standard optically thick and geometrically thin disk solution existing in the innermost regions. We have fitted the spectra of the two black hole sources with the one-temperature models, Sgr A* and XTE J1118+480, which have been examined successfully with two-temperature models. It is found that the one-temperature models do not provide acceptable fits to the multi-wavelength spectra of Sgr A* nor to XTE J1118+480 as a result of the higher temperatures characteristic of the one-temperature models. It is concluded that the thermal coupling of ions and electrons cannot be fully effective and that a two-temperature description is required.

We present 6 cm Very Large Array observations of the Greene & Ho sample of 19 low-mass active galaxies with high accretion rates. This is one of the only studies of a uniform sample of narrow-line Seyfert 1 (NLS1) galaxies with such high sensitivity and resolution. Although we detect only one source, the entire sample is very radio quiet down to strong limits. GH 10 was found to have a radio power of 8.5 × 10²¹ W Hz^-1 and a ratio R ≡ f_{6 cm}/f_{4400 Å} of 2.8. The 3 σ upper limits for the remaining nondetections correspond to radio powers from 3 × 10²⁰ to 8 × 10²¹ W Hz^-1 and 0.47 < R < 9.9. Stacking all nondetections yields an even stronger upper limit of R ≤ 0.27. An assessment of existing observations in the literature confirms our finding that NLS1s are consistently radio-quiet, with a radio-loud fraction of 0%-6%, which is significantly lower than the 10%-20% observed in the general quasar population. By analogy with stellar mass black holes, we argue that AGNs undergo a state transition at L_bol/L_Edd ≈ 0.01. Below this value a radiatively inefficient accretion flow effectively drives an outflow, which disappears when the flow turns into an optically thick, geometrically thin disk, or a radiation pressure-dominated slim disk at still higher L_bol/L_Edd.

We perform local numerical experiments to investigate the nonlinear stability of thin, radially stratified disks. We demonstrate the presence of radial convective instability when the disk is nearly in uniform rotation and show that the net angular momentum transport is slightly inward, consistent with previous investigations of vertical convection. We then show that a convectively unstable equilibrium is stabilized by differential rotation. Convective instability is determined by the Richardson number Ri ≡ N/(Ω)², where N_r is the radial Brunt-Väisälä frequency and Ω is the shear rate. Classical convective instability in a nonshearing medium (Ri → -∞) is suppressed when Ri ≳ -1; i.e., when the shear rate becomes greater than the growth rate. Disks with a nearly Keplerian rotation profile and radial gradients on the order of the disk radius have Ri ≳ -0.01 and are therefore stable to local nonaxisymmetric disturbances. One implication of our results is that the "baroclinic" instability recently claimed by Klahr & Bodenheimer is either global or nonexistent. We estimate that our simulations would detect any genuine growth rate ≳0.0025Ω.

An observation of the H₂O megamaser galaxy IC 2560 with the Chandra X-Ray Observatory reveals a complex spectrum composed of soft X-ray emission due to multitemperature thermal plasma and a hard continuum with strong emission lines. The continuum is most likely a Compton reflection (reprocessing) of primary emission that is completely absorbed at least up to 7 keV. The lines can be identified with fluorescence from Si, S, and Fe in the lowest ionization stages. The equivalent widths of the Si and S lines are broadly compatible with those anticipated for reprocessing by optically thick cold plasma of solar abundances, while the large equivalent width of the Fe line requires some overabundance of iron. A contribution to the line from a transmitted component cannot be ruled out, but the limits on the strength of the Compton shoulder make it less likely. From the bolometric luminosity of the nuclear region, we infer that the source radiates at 1%-10% of its Eddington luminosity for an adopted central mass of 3 × 10⁶M_☉. The overall spectrum is consistent with the hypotheses that the central engines powering the detected megamasers in accretion disks are obscured from direct view by the associated accretion disk material itself and that there is a correlation between the occurrence of megamaser emission and Compton-thick absorption columns. For the 11 known galaxies with both column density measurements and maser emission believed to arise from accretion disks, eight AGNs are Compton thick.

The origin and configuration of the gas that emits broad lines in Type I active galactic nuclei is not established yet. The lack of small-scale structure in the broad emission-line profiles is consistent with either a smooth gas flow or a clumped flow with many small clouds. An attractive possibility for the origin of many small clouds is the atmospheres of bloated stars, an origin that also provides a natural mechanism for the cloud confinement. Earlier studies of the broad-line profiles have already put strong lower limits on the minimum number of such stars, but these limits are sensitive to the assumed width of the lines produced by each cloud. Here we revisit this problem using high-resolution Keck spectra of the Hα line in NGC 4395, which has the smallest known broad-line region (~10¹⁴ cm). Only a handful of the required bloated stars (each having r_* ≈ 10¹⁴ cm) could fit into the broad-line region of NGC 4395, yet the observed smoothness of the Hα line implies a lower limit of ~10⁴-10⁵ on the number of discrete clouds. This conclusively rules out the bloated-stars scenario, regardless of any plausible line-broadening mechanisms. The upper limit on the size of the clouds is ~10¹² cm, which is comparable to the size implied by photoionization models. This strongly suggests that gas in the broad-line region is structured as a smooth rather than a clumped flow, most likely in a rotationally dominated thick disklike configuration. However, it remains to be clarified why such a smooth, gravity-dominated flow generates double-peaked emission lines only in a small fraction of active galactic nuclei.

We present new (B, I) photometry for the globular cluster systems in eight brightest cluster galaxies (BCGs), obtained with the ACS/WFC camera on the Hubble Space Telescope. In the very rich cluster systems that reside within these giant galaxies, we find that all have strongly bimodal color distributions that are clearly resolved by the metallicity-sensitive (B - I) index. Furthermore, the mean colors and internal color range of the blue subpopulation are remarkably similar from one galaxy to the next, to well within the ±0.02-0.03 mag uncertainties in the foreground reddenings and photometric zero points. By contrast, the mean color and internal color range for the red subpopulation differ from one galaxy to the next by twice as much as the blue population. All the BCGs show population gradients, with much higher relative numbers of red clusters within 5 kpc of their centers, consistent with their having formed at later times than the blue, metal-poor population. A striking new feature of the color distributions emerging from our data is that for the brightest clusters (M_I < -10.5) the color distribution becomes broad and less obviously bimodal. This effect was first noticed by Ostrov et al. and Dirsch et al. for the Fornax giant NGC 1399; our data suggest that it may be a characteristic of many BCGs and perhaps other large galaxies. Our data indicate that the blue (metal-poor) clusters brighter than M_I ≃ -10 become progressively redder with increasing luminosity, following a mass/metallicity scaling relation Z ~ M^0.55. A basically similar relation has been found for M87 by Strader et al. (2005). We argue that these GCS characteristics are consistent with a hierarchical-merging galaxy formation picture in which the metal-poor clusters formed in protogalactic clouds or dense starburst complexes with gas masses in the range 10⁷-10¹⁰M_☉, but where the more massive clusters on average formed in bigger clouds with deeper potential wells where more preenrichment could occur.

We present a two-pronged approach to the formation of early-type galaxies, using a sample of 18 galaxies at 0.5 ≲ z ≲ 1 from the HST ACS Ultra Deep Field and GRAPES surveys: (1) We combine slitless low-resolution spectroscopy from the GRAPES data set with simple models of galaxy formation to explore their star formation histories. (2) We also perform an analysis of their surface brightness distribution with the unprecedented details provided by the ACS superb angular resolution and photometric depth. Our spectroscopic analysis reveals that their stellar populations are rather homogeneous in age and metallicity and formed at redshifts z_F ~ 2-5. Evolving them passively, they become practically undistinguishable from elliptical galaxies at z = 0. Also, their isophotal shapes appear very similar to those observed for nearby elliptical galaxies, in that the percentages of disky and boxy galaxies at z ~ 1 are close to the values measured at z = 0. Moreover, we find that the isophotal structure of z ~ 1 early-type galaxies obeys the correlations already observed among nearby elliptical galaxies; i.e., disky elliptical galaxies have generally higher characteristic ellipticities, and boxy elliptical galaxies have larger half-light radii and are brighter in the rest-frame B band. In this respect then, no significant structural differences are seen for elliptical galaxies between z = 0 and 1. Exception can be possibly made for the a₃/a parameter, which is larger at z ~ 1 than usually measured at z = 0. The a₃/a parameter measures the deviations from a pure elliptical isophote, which are not symmetric with respect to the galaxy center, as in the case of dust features and most notably of clumps. Blue clumps have been detected in nearly 50% of the z ~ 1 early-type galaxies; their photometry is suggestive of young star clusters or dwarf irregulars if they are assumed to be at the same redshift as their host galaxies. We speculate that these clumps may represent recent accretion episodes and that they could be a way to produce blue cores if their dynamical time is such for them to rapidly sink to the galaxy center.

Using the SHARC-II camera at the Caltech Submillimeter Observatory to obtain 350 μm images of sources detected with the MIPS instrument on Spitzer, we have discovered a remarkable object at z = 1.325 ± 0.002 with an apparent far-infrared luminosity of 3.2(±0.7) × 10¹³L_☉. Unlike other z > 1 sources of comparable luminosity selected from mid-IR surveys, MIPS J142824.0+352619 lacks any trace of AGN activity, and is likely a luminous analog of galaxies selected locally by IRAS, or at high redshift in the submillimeter. This source appears to be lensed by a foreground elliptical galaxy at z = 1.034, although the amplification is likely modest (≲10). We argue that the contribution to the observed optical/near-IR emission from the foreground galaxy is small, and hence are able to present the rest-frame UV through radio spectral energy distribution of this galaxy. Due to its unusually high luminosity, MIPS J142824.0+352619 presents a unique chance to study a high-redshift dusty starburst galaxy in great detail.

We present an analytical model for the hydrodynamic outflow from the disk of a starburst galaxy. The model is used to calculate the cosmic-ray propagation and the radio intensity distribution in the nuclear starburst region of NGC 253. We find that the cosmic-ray energy production rate of the central 600 pc of NGC 253 is about 3 × 10⁴¹ ergs s^-1, which is about 15% of the total mechanical supernova power. For this inner region, we estimate a terminal outflow velocity of 900 km s^-1 and a mass-loss rate of = 2-4 M_☉ yr^-1.

Peaks and lulls in the star formation rate (SFR) over the history of the Galaxy produce plateaus and declines in the present-day mass function (PDMF) where the main-sequence lifetime overlaps the age and duration of the SFR variation. These PDMF features can be misinterpreted as the form of the intrinsic stellar initial mass function (IMF) if the star formation rate is assumed to be constant or slowly varying with time. This effect applies to all regions that have formed stars for longer than the age of the most massive stars, including OB associations, star complexes, and especially galactic field stars. Related problems may apply to embedded clusters. Evidence is summarized for temporal SFR variations from parsec scales to entire galaxies, all of which should contribute to inferred IMF distortions. We give examples of various star formation histories to demonstrate the types of false IMF structures that might be seen. These include short-duration bursts, stochastic histories with lognormal amplitude distributions, and oscillating histories with various periods and phases. The inferred IMF should appear steeper than the intrinsic IMF over mass ranges where the stellar lifetimes correspond to times of decreasing SFRs; shallow portions of the inferred IMF correspond to times of increasing SFRs. If field regions are populated by dispersed clusters and defined by their low current SFRs, then they should have steeper inferred IMFs than the clusters. The SFRs required to give the steep field IMFs in the LMC and SMC are determined. Structure observed in several determinations of the Milky Way field star IMF can be accounted for by a stochastic and bursty star formation history.

We investigate the physical properties of the interstellar medium (ISM) in the merging pair of galaxies known as the Antennae (NGC 4038/4039), using the deep co-added ~411 ks Chandra ACIS-S data set. The method of analysis and some of the main results from the spectral analysis, such as metal abundances and their variations from ~0.2 to ~20-30 times solar, are described in Paper I (Baldi et al.). In the present paper we investigate in detail the physics of the hot emitting gas, deriving measures for the hot gas mass (~10⁷M_☉), cooling times (10⁷-10⁸ yr), and pressure (3.5 × 10^-11-2.8 × 10^-10 dyne cm^-2). In at least one of the two nuclei (NGC 4038), the hot gas pressure is significantly higher than the CO pressure, implying that shock waves may be driven into the CO clouds. Comparison of the metal abundances with the average stellar yields predicted by theoretical models of SN explosions points to SNe of Type II as the main contributors of metals to the hot ISM. There is no evidence of any correlation between radio-optical star formation indicators and the measured metal abundances. Although due to uncertainties in the average gas density we cannot exclude that mixing may have played an important role, the short time required to produce the observed metal masses (≲2 Myr) suggests that the correlations are unlikely to have been destroyed by efficient mixing. More likely, a significant fraction of Type II SN ejecta may be in a cool phase, in grains, or escaping in hot winds. In each case, any such fraction of the ejecta would remain undetectable with soft X-ray observations.

We present here the first detailed study of a giant radio galaxy of the fat-double type. The lobes of the double radio galaxy SGRS J0515-8100 have transverse widths that are 1.3 times their extent from the center, their surface brightness is the lowest among known giant radio sources, and the lobes have relatively steep radio spectra. We infer that these wide lobes were created as a result of a highly variable and intermittent jet whose axis direction also varied significantly: the fat-double lobes in this giant radio source are a result of the ejection and deposition of synchrotron plasma over a wide range of angles over time rather than the expansion of relic lobes. In addition, the optical host shows evidence for an ongoing galaxy-galaxy interaction. SGRS J0515-8100 supports the hypothesis that interactions with companions might perturb the inner accretion disk that produces and sustains the jets at the centers of active galactic nuclei. As a result, it appears unnecessary to invoke black hole coalescence to explain such morphologies, implying that the corresponding event rates predicted for gravitational wave detectors may be overestimates.

We present imaging Fabry-Perot observations of Hα emission in the nearly edge-on spiral galaxy NGC 5775. We have derived a rotation curve and a radial density profile along the major axis by examining position-velocity (PV) diagrams from the Fabry-Perot data cube, as well as a CO 2-1 data cube from the literature. PV diagrams constructed parallel to the major axis are used to examine changes in azimuthal velocity as a function of height above the midplane. The results of this analysis reveal the presence of a vertical gradient in azimuthal velocity. The magnitude of this gradient is approximately 1 km s^-1 arcsec^-1, or about 8 km s^-1 kpc^-1, although a higher value of the gradient may be appropriate in localized regions of the halo. The evidence for an azimuthal velocity gradient is much stronger for the approaching half of the galaxy, although earlier slit spectra are consistent with a gradient on both sides. There is evidence for an outward radial redistribution of gas in the halo. The form of the rotation curve may also change with height, but this is not certain. We compare these results with those of an entirely ballistic model of a disk-halo flow. The model predicts a vertical gradient in azimuthal velocity that is shallower than the observed gradient, indicating that an additional mechanism is required to further slow the rotation speeds in the halo.

We present an XMM-Newton X-ray observation of the X-ray-bright E2 elliptical galaxy NGC 4649. In addition to bright diffuse emission, we resolve 158 discrete sources, ~50 of which are likely to be LMXBs associated with NGC 4649. We find evidence for variability in three sources between this observation and a previous Chandra observation. Additionally, we detect five sources that were not detected with Chandra despite its better detection limit, suggesting that these sources have since brightened. The total X-ray spectrum of the resolved sources is well fit by a hard power law, while the diffuse spectrum requires a hard and a soft component, presumably due to the relatively soft diffuse gas and the harder unresolved sources. A deprojection of the diffuse emission revealed a radial temperature gradient that is hot in the center, drops to a minimum at about 20''-50'' (1.6-4.1 kpc), and rises again in the outer regions. The diffuse emission appears to require a two-temperature model with heavy-element abundance ratios that differ from the solar values. We have verified the existence of faint radial features extending out from the core of NGC 4649 that had previously been seen with Chandra. The fingers are morphologically similar to radial features seen in hydrodynamic simulations of cooling flows in elliptical galaxies, and although their other properties do not match the predictions of the particular simulations used, we conclude that the radial fingers might be due to convective motions of hot outflowing gas and cooler inflowing gas. We also find evidence for a longer, previously undetected filament that extends to the northeastern edge of NGC 4649. The diffuse gas in the region of the filament appears to have a lower temperature and may also have a higher abundance as compared to nearby regions. There also appears to be an excess of X-ray sources along the filament, although the excess is not statistically significant. We conclude that the filament may be the result of a tidal interaction, possibly with NGC 4647, although more work is necessary to verify this conclusion.

We present long-slit optical spectroscopy of 67 H II regions in 21 dwarf irregular galaxies to investigate the enrichment of oxygen, nitrogen, neon, sulfur, and argon in low-mass galaxies. Oxygen abundances are obtained via direct detection of the temperature-sensitive emission lines for 25 H II regions; for the remainder of the sample, oxygen abundances are estimated from strong-line calibrations. The direct abundance determinations are compared to the strong-line abundance calibrations of both McGaugh and Pilyugin. While the McGaugh calibration yields a statistical offset of 0.07 dex, the photoionization model grid traces the appropriate isometallicity contour shape in the R₂₃-O₃₂ diagnostic diagram. In contrast, while the Pilyugin calibration yields a negligible statistical offset, the residuals in this strong-line calibration method are correlated with ionization parameter. Thus, these observations indicate that oxygen abundances will be overestimated by the p-method for H II regions with low-ionization parameters. Global oxygen and nitrogen abundances for this sample of dwarf irregular galaxies are examined in the context of open- and closed-box chemical evolution models. While several galaxies are consistent with closed-box chemical evolution, the majority of this sample has an effective yield ~ of the expected yield for a constant star formation rate and Salpeter IMF, indicating that either outflow of enriched gas or inflow of pristine gas has occurred. The effective yield strongly correlates with M_H/L_B in the sense that gas-rich galaxies are more likely to be closed systems. However, the effective yield does not appear to correlate with other global parameters such as dynamical mass, absolute magnitude, star formation rate, or surface brightness. In addition, open and closed systems are not identified easily in other global abundance measures; for example, the observed correlation between luminosity and metallicity is consistent with other recent results in the literature. A correlation is found between the observed nitrogen-to-oxygen ratio and the color of the underlying stellar population; redder dwarf irregular galaxies have higher N/O ratios than blue dwarf irregular galaxies. The relative abundance ratios are interpreted in the context of delayed release of nitrogen and varied star formation histories.

We present a measurement of the microlensing optical depth toward the Galactic bulge based on 4 years of the OGLE-II survey. We consider only bright sources in the extended red clump giant (RCG) region of the color-magnitude diagram, in 20 bulge fields covering ~5 deg² between 0° < l < 3° and -4° < b < -2°. Using a sample of 32 events we find τ = 2.55 × 10^-6 at (l, b) = (116, - 275). Taking into account the measured gradient along the Galactic latitude b, τ = [(4.48 ± 2.37) + (0.78 ± 0.84) × b] × 10^-6, this value is consistent with previous measurements using RCG sources and recent theoretical predictions. We determine the microlensing parameters and select events using a model light curve that allows for flux blending. Photometric quality delivered by difference image analysis (DIA) combined with the 13 median seeing of the OGLE-II images are sufficient to constrain and reject the majority of strong blends. We find that ~38% of the OGLE-II events that appear to have RCG sources are actually due to much fainter stars blended with a bright companion. We show explicitly that model fits without blending result in similar τ estimates through partial cancellation of contributions from higher detection efficiency, underestimated timescales, and a larger number of selected events. The near cancellation of the optical depth bias and the fact that microlensing event selection based on models without blending discriminates against blends have been utilized by previous analyses based on RCG sources. The latter approach, however, leads to biased timescale distributions and event rates. Consequently, microlensing studies should carefully consider source confusion effects even for bright stars.

We obtained the first view in H¹³CO⁺J = 1-0 and a high-resolution map in thermal SiO lines of G0.11-0.11, which is a molecular cloud situated between the Galactic Center radio arc and Sgr A. From a comparison with previous line observations, we found that the H¹³CO⁺J = 1-0 line is optically thin, whereas the thermal SiO lines are optically thick. The line intensity in H¹³CO⁺J = 1-0 shows that the cloud has a large column density, up to N(H₂) = (6-7) × 10²³ cm^-2, which corresponds to about 640-740 mag in A_V or 10-12 mag in A_{25 μm}. The estimated column density is the largest known of any even in the Galactic center region. We conclude from the intensity ratio of SiO J = 1-0 to CS J = 1-0 that emitting gas is highly inhomogeneous for SiO abundance on a scale smaller than the beam width ~35''.

A 3 × 3 map of the Galactic center was made at 9' resolution and 10' spacing in the CH ²Π_1/2, J = 1/2, F = 1-1 transition at 3335 MHz. The CH emission shows a velocity extent that is close to that of the CO(1-0) line, but the CH line profiles differ markedly from the CO. The 3335 MHz CH transition primarily traces low-density molecular gas in the high-density, high-pressure environment of the Galactic center interstellar medium. If the empirical N(CH)-N(H₂) relation derived for molecular clouds in the disk is valid at the Galactic center, then our observations indicate that the mass of the low-density component within ~30 pc of the Galactic center is ~9 × 10⁶M_☉. The CO-H₂ conversion factor obtained for the low-density gas in the mapped region is greater than that thought to apply to the dense molecular gas at the Galactic center. The CH spectra show evidence of emission from molecular clouds along the line of sight in both the foreground and the background. The scale height of these clouds ranges from 27 to 109 pc, consistent with previous work based on observations of molecular clouds in the inner Galaxy.

Highly sensitive imaging observations of the Galactic center (GC) at high energies with an angular resolution of order 10' is a very recent development in the field of high-energy astrophysics. The IBIS/ISGRI imager on the INTEGRAL observatory detected for the first time a hard X-ray source, IGR J17456-2901, located within 1' of Sagittarius A* (Sgr A*) over the energy range 20-100 keV. Here we present the results of a detailed analysis of approximately 7 × 10⁶ s of observations of the GC obtained since the launch of INTEGRAL in 2002 October. Two years and an effective exposure of 4.7 × 10⁶ s have allowed us to obtain more stringent positional constraints on this high-energy source and to construct its spectrum in the range 20-400 keV. Furthermore, by combining the ISGRI spectrum with the total X-ray spectrum corresponding to the same physical region around Sgr A* from XMM-Newton data collected during part of the γ-ray observations, we constructed and present the first accurate wideband high-energy spectrum for the central arcminutes of the Galaxy. Our complete and updated analysis of the emission properties of the INTEGRAL source shows that it is faint but persistent with no variability above 3 σ, contrary to what was alluded to in our first paper. This result, in conjunction with the spectral characteristics of the soft and hard X-ray emission from this region, suggests that the source is most likely not pointlike but rather that it is a compact yet diffuse nonthermal emission region. The centroid of IGR J17456-2901 is estimated to be R.A. = 17^h45^m425, decl. = -28°59'28'' (J2000.0), offset by 1' from the radio position of Sgr A* and with a positional uncertainty of 1'. Its 20-400 keV luminosity at 8 kpc is L = (5.37 ± 0.21) × 10³⁵ ergs s^-1. A 3 σ upper limit on the flux at the electron-positron annihilation energy of 511 keV from the direction of Sgr A* is set at 1.9 × 10^-4 photons cm^-2 s^-1. Very recently, the HESS collaboration presented the detection of a source of ~TeV γ-rays also located within an arcminute of Sgr A*. We present arguments in favor of an interpretation that the photons detected by INTEGRAL and HESS arise from the same compact region of diffuse emission near the central black hole and that the supernova remnant Sgr A East could play an important role as a contributor of very high energy γ-rays to the overall spectrum from this region. There is also evidence for hard emission from a region located between the central black hole and the radio arc near l ~ 01 along the Galactic plane and known to contain giant molecular clouds.

We use linear analysis to examine the effect of cosmic rays (CRs) on the Parker-Jeans instability of magnetized self-gravitating gaseous disks. We adopt a slab equilibrium model in which the gravity (including self-gravity) is perpendicular to the midplane and the magnetic field lies along the slab. CRs are described as a fluid, and diffusion is considered only along magnetic field lines. The linearized equations are solved numerically. The system is susceptible to Parker-Jeans instability. In general, the system is less unstable when the CR diffusion coefficient is smaller (i.e., the coupling between the CRs and plasma is stronger). The system is also less unstable if the CR pressure is larger. This is reminiscent of the fact that Jeans instability and Parker instability are less unstable when the gas pressure is larger (or temperature is higher). Moreover, for a large CR diffusion coefficient (or a small CR pressure), perturbations parallel to the magnetic field are more unstable than those perpendicular to it. The other governing factor on the growth rate of the perturbations in different directions is the thickness of the disk or the strength of the external pressure on the disk. In fact, this is the determining factor in some parameter regimes.

The carrier of the dust-associated photoluminescence process causing ERE in many dusty interstellar environments remains unidentified. Several competing models are more or less able to match the observed broad, unstructured ERE band. We now constrain the character of the ERE carrier further by determining the wavelengths of the radiation that initiates the ERE. Using the imaging capabilities of the HST, we have resolved the width of narrow ERE filaments appearing on the surfaces of externally illuminated molecular clouds in the bright reflection nebula NGC 7023 and compared them with the depth of penetration of radiation of known wavelengths into the same cloud surfaces. We identify photons with wavelengths shortward of 118 nm as the source of ERE initiation, not to be confused with ERE excitation, however. There are strong indications from the well-studied ERE in the Red Rectangle Nebula and in the high-|b| Galactic cirrus that the photon flux with wavelengths shortward of 118 nm is too small to actually excite the observed ERE, even with 100% quantum efficiency. We conclude, therefore, that ERE excitation results from a two-step process. The first, involving far-UV photons with E > 10.5 eV, leads to the creation of the ERE carrier, most likely through photoionization or photodissociation of an existing precursor. The second, involving more abundant near-UV/optical photons, consists of the optical pumping of the previously created carrier, followed by subsequent deexcitation via photoluminescence. The latter process can occur many times for a single particle, depending upon the lifetime of the ERE carrier in its active state. While none of the previously proposed ERE models can match these new constraints, we note that under interstellar conditions most PAH molecules are ionized to the dication stage by photons with E > 10.5 eV and that the electronic energy level structure of PAH dications is consistent with fluorescence in the wavelength band of the ERE. Therefore, PAH dications deserve further study as potential carriers of the ERE.

We model the observed size and brightness of the VLBA radio core of the jet in Cygnus X-1 to derive an expression for the jet power as a function of basic jet parameters. We apply this expression to recent constraints on the jet power from observations of a large-scale shocked shell around the source by Gallo and coworkers, which leads us to a set of alternative conclusions: either (1) the jet contains large amounts of protons (≥2000 protons per radio-emitting electron), (2) it has a very low volume filling factor of f ≲ 3 × 10^-5, (3) the steady, radio-emitting VLBA jet is not the source of the kinetic energy powering the ISM shell, or (4) its asymptotic behavior differs fundamentally from a broad set of plausible analytic jet models.

Submillimeter Array observations of Orion KL at ~1'' resolution in the 440 μm/690 GHz band reveal new insights about the continuum and line emission of the region. The 440 μm continuum flux density measurement from source I allows us to differentiate among the various proposed physical models. Source I can be well modeled by a "normal" protostellar spectral energy distribution (SED) consisting of a proton-electron free-free emission component at low frequencies and a strong dust component in the submillimeter bands. Furthermore, we find that the protostellar object SMA1 is clearly distinct from the hot core. The nondetection of SMA1 at centimeter and infrared wavelengths suggests that it may be one of the youngest sources in the entire Orion KL region. The molecular line maps show emission mainly from source I, SMA1, and the hot core peak position. An analysis of the CH₃CN(37_K-36_K) K-ladder (K = 0, ... ,3) indicates a warm gas component of the order of 600 ± 200 K. In addition, we detect a large fraction (~58%) of unidentified lines and discuss the difficulties of line identification at these frequencies.

Here we made multiconstituent (gas/dust/field), homogeneous maps of a length of a vertical filament including DR 21(OH) proper. We observed at a radial velocity around -3.5 km s^-1 the ¹²CO, ¹³CO, and C¹⁸O gas in the J = 3-2 lines, as well as the thermal dust continuum near 850 μm and the dust's linear polarization properties, all near the same wavelength (850 μm) and with the same angular resolution (14'') using the same antenna (James Clerk Maxwell Telescope [JCMT]). We deduce the energy components in the cool filament, and we compute the pressure components there and those in the surrounding region. The vertical filament has a total pressure composed about equally of turbulent energy and magnetic energy. The magnetic field in the vertical filament appears to go across the filament and has an estimated strength of 100-200 μG in the sky plane. Near the map center, we find a low-velocity outflow from DR 21(OH), with a pattern of blue southeast gas and red northwest gas. For the central DR 21(OH) protostar, our JCMT polarimetric data allow us to infer a median "plane-of-sky field" of 780 μG oriented mainly east-west (as seen with the JCMT beam of 14''), which is comparable to the published Zeeman data along the line of sight (-530 μG).

We examine the "puffed-up inner disk" model proposed by Dullemond, Dominik, & Natta for explaining the near-IR excess radiation from Herbig Ae/Be stars. Detailed model computations show that the observed near-IR excess requires more hot dust than is contained in the puffed-up disk rim. The rim can produce the observed near-IR excess only if its dust has perfectly gray opacity, but such dust is in conflict with the observed 10 μm spectral feature. We find that a compact (~10 AU), tenuous (τ_V ≲ 0.4), dusty halo around the disk inner regions contains enough dust to readily explain the observations. Furthermore, this model also resolves the puzzling relationship noted by Monnier & Millan-Gabet between luminosity and the interferometric inner radii of disks.

We use three-dimensional radiative transfer models to show the effects of clumpy circumstellar material on the observed infrared colors of high-mass stars embedded in molecular clouds. We highlight differences between three-dimensional clumpy and one-dimensional smooth models that can affect the interpretation of data. We discuss several important properties of the emergent spectral energy distribution (SED). More near-infrared light (scattered and direct from the central source) can escape than in smooth one-dimensional models. The near- and mid-infrared SED of the same object can vary significantly with viewing angle, depending on the clump geometry along the sight line. Even the wavelength-integrated flux can vary with angle by more than a factor of 2. Objects with the same average circumstellar dust distribution can have very different near- and mid-IR SEDs, depending on the clump geometry and the proximity of the most massive clump to the central source. Although clumpiness can cause similar objects to have very different SEDs, there are some observable trends. Near- and mid-infrared colors are sensitive to the weighted average distance of clumps from the central source and to the magnitude of clumpy density variations (smooth-to-clumpy ratio). Far-infrared emission remains a robust measure of the total dust mass. We present simulated SEDs, colors, and images for 2MASS and Spitzer filters. We compare them to observations of some ultracompact H II regions and find that three-dimensional clumpy models fit better than smooth models. In particular, clumpy models with fractal dimensions in the range 2.3-2.8, smooth-to-clumpy ratios of ≲50%, and density distributions with shallow average radial density profiles fit the SEDs best (⟨ρ⟩ ∝ r^α, - 1.0 < α < 0.0).

We identify the fading X-ray afterglow of GRB 001025A from XMM-Newton observations obtained 1.9-2.3 days, 2 yr, and 2.5 yr after the burst. The nondetection of an optical counterpart to an upper limit of R = 25.5, 1.20 days after the burst, makes GRB 001025A a "dark" burst. Based on the X-ray afterglow spectral properties of GRB 001025A, we argue that some bursts appear optically dark because their afterglow is faint and their cooling frequency is close to the X-ray band. This interpretation is applicable to several of the few other dark bursts where the X-ray spectral index has been measured. The X-ray afterglow flux of GRB 001025A is an order of magnitude lower than for typical long-duration gamma-ray bursts. The spectrum of the X-ray afterglow can be fitted with an absorbed synchrotron emission model, an absorbed thermal plasma model, or a combination thereof. For the latter, an extrapolation to optical wavelengths can be reconciled with the R-band upper limit on the afterglow, without invoking any optical circumburst absorption, provided the cooling frequency is close to the X-ray band. Alternatively, if the X-ray afterglow is due to synchrotron emission only, 7 mag of extinction in the observed R-band is required to meet the R-band upper limit, making GRB 001025A much more obscured than bursts with detected optical afterglows. Based on the column density of X-ray-absorbing circumburst matter, an SMC gas-to-dust ratio is insufficient to produce this amount of extinction. The X-ray tail of the prompt emission enters a steep temporal decay excluding that the tail of the prompt emission is the onset of the afterglow. To within the astrometric uncertainty, this afterglow was coincident with an extended object, seen in a deep VLT R-band image, which we identify as the likely host galaxy of GRB 001025A.

We present the results from a Hubble Space Telescope ACS study of the supernovae (SNe) associated with gamma-ray bursts (GRBs) 040924 (z = 0.86) and 041006 (z = 0.71). We find evidence that both GRBs were associated with an SN 1998bw-like supernova dimmed by ~1.5 and ~0.3 mag, respectively, making GRB 040924 the faintest GRB-associated SN ever detected. We study the luminosity dispersion in GRB/XRF-associated SNe and compare to local Type Ibc SNe from the literature. We find significant overlap between the two samples, suggesting that GRB/XRF-associated SNe are not necessarily more luminous and do not necessarily produce more ⁵⁶Ni than local SNe. Based on the current (limited) data sets, we find that the two samples may share a similar ⁵⁶Ni production mechanism.

We present the spectral evolution, light curve, and corresponding interpretation for the "normal-bright" Type Ia supernova 2005cg discovered by ROTSE-IIIc. The host is a low-luminosity (M_r = -16.75) blue galaxy with strong indications of active star formation and an environment similar to that expected for SNe Ia at high redshifts. Early-time (t ~ -10 days) optical spectra obtained with the HET reveal an asymmetric, triangular-shaped Si II absorption feature at about 6100 Å with a sharp transition to the continuum at a blueshift of about 24,000 km s^-1. By 4 days before maximum, the Si II absorption feature becomes symmetric with smoothly curved sides. Similar Si II profile evolution has previously been observed in other supernovae and is predicted by some explosion models, but its significance has not been fully recognized. Although the spectra predicted by pure deflagration and delayed detonation models are similar near maximum light, they predict qualitatively different chemical abundances in the outer layers and thus give qualitatively different spectra at the earliest phases. The Si line observed in SN 2005cg at early times requires the presence of burning products at high velocities, and the triangular shape is likely to be formed in an extended region of slowly declining Si abundance that characterizes delayed detonation models. The spectra show a high-velocity Ca II IR feature that coincides in velocity space with the Si II cutoff. This supports the interpretation that the Ca II is formed when the outer layers of the SN ejecta sweep up about 5 × 10^-3M_☉ of material within the progenitor system. We compare our results with other "Branch-normal" SNe Ia with early-time spectra, namely, SN 2003du, 1999ee, and 1994D. Although the expansion velocities based on their Si II absorption minima differ, all show triangular-shaped profiles and velocity cutoffs between 23,000 and 25,000 km s^-1, which are consistent with the Doppler shifts of their respective high-velocity Ca II IR features. SN 1990N-like objects, however, showed distinctly different behavior, which may suggest separate progenitor subclasses.

We detected a nearby (d ≃ 360 pc), old (τ ≃ 5 Myr) pulsar B1133+16 with Chandra. The observed pulsar's flux is (0.8 ± 0.2) × 10^-14 ergs cm^-2 s^-1 in the 0.5-8 keV band. Because of the small number of counts detected, the spectrum can be described by various models. A power-law fit of the spectrum gives a photon index Γ ≈ 2.5 and an isotropic luminosity of 1.4 × 10²⁹ ergs s^-1 in the 0.5-8 keV band, which is about 1.6 × 10^-3 of the spin-down power . The spectrum can also be fitted by a blackbody model with a temperature of ≈2.8 MK and a projected emitting area of ~500 m², possibly a hot polar cap. The X-ray properties of PSR B1133+16 are similar to those of other old pulsars observed in X-rays, particularly the drifting pulsar B0943+10.

We apply the semianalytical analysis of the steady nature of line-driven winds presented in two earlier papers to disk winds driven by the flux distribution of a standard Shakura & Sunyaev disk for typical cataclysmic variable (CV) parameters. Our main conclusion is that a line-driven wind, arising from a steady disk flux distribution of a standard Shakura-Sunyaev disk, is steady. These results are consistent with the steady velocity nature of outflows observationally inferred for both CVs and quasi-stellar objects (QSOs). We find good agreement with the 2.5-dimensional CV disk wind models of Pereyra and collaborators. We find that the wind critical point tends to be closer to the disk surface toward the inner disk regions.

We present X-ray observations of Vega obtained with the Chandra High Resolution Camera and Advanced CCD Imaging Spectrometer. After a total of 29 ks of observation with Chandra, X-rays from Vega remain undetected. We derive upper limits to the X-ray luminosity of Vega as a function of temperature over the range of 10⁵-10⁷ K and find a 99.7% upper limit as low as ~2 × 10²⁵ ergs s^-1 at T = 10^6.2 K. We also compare these new deeper observations with the limit derived from a reanalysis of ROSAT PSPC data. Our X-ray luminosity limit for Vega is still greater than predictions of post-Herbig Ae phase X-rays from the shear dynamo model proposed by Tout & Pringle for a Vega age of 350 Myr. If the age of Vega is closer to 100 Myr, as suggested by some indicators, our X-ray limit is then similar to Tout-Pringle model predictions. Current X-ray observations of Vega are therefore unable to discriminate between different scenarios explaining the X-ray activity of the convectively stable Herbig Ae/Be stars. Further progress is more likely to be achieved through X-ray observations of younger main-sequence early-type A stars, whose conjectured residual post-Herbig Ae phase X-ray activity would be significantly higher.

Oxygen abundances have been derived from the near-IR, high-excitation O I λ7774 triplet in high-resolution, high signal-to-noise ratio spectra of 45 Hyades dwarfs using standard one-dimensional, plane-parallel LTE models. Effective temperatures of the stellar sample range from 4319 to 6301 K, and the derived relative O abundances as a function of T_eff evince a trichotomous morphology. At T_eff > 6100 K, there is evidence of an increase in the O abundances with increasing T_eff, consistent with non-LTE (NLTE) predictions. At intermediate T_eff (5450 K ≤ T_eff ≤ 6100 K), the O abundances are flat, and star-to-star values are in good agreement, having a mean value of [O/H] = +0.25 ± 0.02; however, systematic errors at the ≲0.10 dex level might exist. The O abundances for stars with T_eff ≤ 5450 K show a striking increase with decreasing T_eff, in stark contrast to expectations and canonical NLTE calculations. The cool Hyades triplet results are compared to those recently reported for dwarfs in the Pleiades cluster and the UMa moving group; qualitative differences between the trends observed in these stellar aggregates point to a possible age-related diminution of triplet abundance trends in cool open cluster dwarfs. Correlations with age-related phenomena, i.e., chromospheric activity and photospheric spots, faculae, and/or plages, are investigated. No correlation with Ca II H+K chromospheric activity indicators is observed. Multicomponent LTE "toy" models have been constructed in order to simulate photospheric temperature inhomogeneities that could arise from the presence of starspots, and we demonstrate that photospheric spots are a plausible source of the triplet trends among the cool dwarfs.

We report g, V, and r photometric time series of HD 149026 spanning predicted times of transit of the Saturn-mass planetary companion, which was recently discovered by Sato and collaborators. We present a joint analysis of our observations and the previously reported photometry and radial velocities of the central star. We refine the estimate of the transit ephemeris to T_c = (2,453,527.87455) + (2.87598)N (HJD). Assuming that the star has a radius of 1.45 ± 0.10 R_☉ and a mass of 1.30 ± 0.10 M_☉, we estimate the planet radius to be (0.726 ± 0.064)R_Jup, which implies a mean density of 1.07 g cm^-3. This density is significantly greater than predicted for models that include the effects of stellar insolation and in which the planet has only a small core of solid material. Thus, we confirm that this planet likely contains a large core and that the ratio of core mass to total planet mass is more akin to that of Uranus and Neptune than to either Jupiter or Saturn.

We study the energy and helicity injected into the corona by the slow motion of photospheric source regions. A previous study compared these quantities in a simple quadrupolar configuration modeled by a quasi-static, line-tied MHD simulation and a minimum current corona (MCC). The MCC provides a lower bound for the coronal magnetic free energy by quantifying the coronal linkages (flux domains) between discrete photospheric source regions; the chosen configuration contains four flux domains and one separator. The MCC analysis can be extended by decomposing each source region into smaller ones, increasing the number of flux domains and separators. This creates a hierarchy of topological features that asymptotically approaches a line-tied model. We demonstrate the hierarchical approach using two octopolar decompositions of the previously studied quadrupole. One of these has helicity and free energy significantly closer to those of the line-tied experiment; this is primarily due to the interweaving of lower level flux domains approximating the self-helicity of a rotating region. The other decomposition does not allow such interweaving and has helicity and free energy comparable to the quadrupolar MCC configuration.

We study the acceleration in solar flares of ³He and ⁴He from a thermal background by parallel-propagating plasma waves with a general broken power-law spectrum. The exact dispersion relation for a cold plasma is used to describe the relevant wave modes, and the Coulomb collision loss and escape processes are included. Under the quasi-linear approximation, the pitch-angle-averaged acceleration time of α-particles is at least 1 order of magnitude longer than that of ³He ions at low energies and starts to approach that of ³He beyond a few tens of keV nucleon^-1. Because their loss and escape times are comparable, the acceleration of ⁴He is suppressed significantly at low energies, and the spectrum of the accelerated α-particles is always softer than that of ³He. Quantitative results depend primarily on the wave generation and damping length scales, the electron plasma to gyrofrequency ratio, and the intensity of turbulence. The model gives a reasonable account of the observed low-energy ³He and ⁴He fluxes and spectra in the impulsive solar energetic particle events observed with the Advanced Composition Explorer. Other acceleration processes and/or stochastic acceleration by other wave modes seem to be required to explain the occasionally observed decrease of ³He to ⁴He ratio at energies beyond a few MeV nucleon^-1.

We outline a general methodology to infer the inductive velocity field vector in solar active regions. For the first time, both the field-aligned and the cross-field velocity components are reconstructed. The cross-field velocity solution accounts for the changes of the vertical magnetic field seen between a pair of successive active region vector magnetograms via the ideal induction equation. The field-aligned velocity is obtained using the Doppler velocity and the calculated cross-field velocity. Solving the ideal induction equation in vector magnetograms measured at a given altitude in the solar atmosphere is an underdetermined problem. In response, our general formalism allows the use of any additional constraint for the inductive cross-field velocity to enforce a unique solution in the induction equation. As a result, our methodology can give rise to new velocity solutions besides the one presented here. To constrain the induction equation, we use a special case of the minimum structure approximation that was introduced in previous studies and is already employed here to resolve the 180° ambiguity in the input vector magnetograms. We reconstruct the inductive velocity for three active regions, including NOAA AR 8210, for which previous results exist. Our solution believably reproduces the horizontal flow patterns in the studied active regions but breaks down in cases of localized rapid magnetic flux emergence or submergence. Alternative approximations and constraints are possible and can be accommodated into our general formalism.

The quiet-Sun photospheric plasma has a variety of magnetic field strengths going from zero to 1800 G. The empirical characterization of these field strengths requires a probability density function (PDF), i.e., a function P(B) describing the fraction of quiet Sun occupied by each field strength B. We show how to combine magnetic field strength measurements based on the Zeeman effect and the Hanle effect to estimate an unbiased P(B). The application of the method to real observations renders a set of possible PDFs, which outline the general characteristics of the quiet-Sun magnetic fields. Their most probable field strength differs from zero. The magnetic energy density is a significant fraction of the kinetic energy of the granular motions at the base of the photosphere (larger than 15% or larger than 2 × 10³ ergs cm^-3). The unsigned flux density (or mean magnetic field strength) has to be between 130 and 190 G. A significant part of the unsigned flux (between 10% and 50%) and of the magnetic energy (between 45% and 85%) are provided by the field strengths larger than 500 G, which, however, occupy only a small fraction of the surface (between 1% and 10%). The fraction of kG fields in the quiet Sun is even smaller, but they are important for a number of reasons. The kG fields still trace a significant fraction of the total magnetic energy, they reach the high photosphere, and they appear in unpolarized light images. The quiet-Sun photosphere has far more unsigned magnetic flux and magnetic energy than the active regions and the network combined.

We present a fitting function to describe the statistics of flux modulations caused by interstellar scintillation. The function models a very general quantity: the cross-correlation of the flux observed from a compact radio source of finite angular size observed at two frequencies and at two positions or times. The formula will be useful for fitting data from sources such as intraday variables and gamma-ray burst afterglows. These sources are often observed at relatively high frequencies (several gigahertz), where interstellar scattering is neither very strong nor very weak, so that asymptotic formulae are inapplicable.

Pupil mapping is a technique whereby a uniformly illuminated input pupil, such as from starlight, can be mapped into a nonuniformly illuminated exit pupil, such that the image formed from this pupil will have suppressed sidelobes, many orders of magnitude weaker than classical Airy ring intensities. Pupil mapping is therefore a candidate technique for coronagraphic imaging of extrasolar planets around nearby stars. Unlike most other high-contrast imaging techniques, pupil mapping is lossless and preserves the full angular resolution of the collecting telescope. So it could possibly give the highest signal-to-noise ratio of any proposed single-telescope system for detecting extrasolar planets. Prior analyses based on pupil-to-pupil ray-tracing indicate that a planet fainter than 10^-10 times its parent star, and as close as about 2λ/D, should be detectable. In this paper we describe the results of careful diffraction analysis of pupil-mapping systems. These results reveal a serious unresolved issue. Namely, high-contrast pupil mappings distribute light from very near the edge of the first pupil to a broad area of the second pupil, and this dramatically amplifies diffraction-based edge effects, resulting in a limiting attainable contrast of about 10^-5. We hope that by identifying this problem, others will provide a solution.

We present calculations of electron capture cross sections in collisions of O²⁺ and N²⁺ with H(1s) for impact energies 0.001 eV < E < 10 keV and the corresponding rate coefficients for temperatures 10² K < T < 10⁵ K. Our molecular close-coupling treatment leads to significant differences from the capture rates usually employed in the modeling of astrophysical plasmas.

This paper describes a very general approach to the calculation of the Zeeman splitting effect produced by an external magnetic field on the rotational levels of diatomic molecules. The method is valid for arbitrary values of the total electronic spin and of the magnetic field strength—that is, it holds for molecular electronic states of any multiplicity and for both the Zeeman and incomplete Paschen-Back regimes. It is based on an efficient numerical diagonalization of the effective Zeeman Hamiltonian, which can incorporate easily all the contributions one may eventually be interested in, such as the hyperfine interaction of the external magnetic field with the spin motions of the nuclei. The reliability of the method is demonstrated by comparing our results with previous ones obtained via formulae valid only for doublet states. We also present results for molecular transitions arising between nondoublet electronic states, illustrating that their Zeeman patterns show signatures produced by the Paschen-Back effect.

We cross-correlate the first-year Wilkinson Microwave Anisotropy Probe (WMAP) data and the diffuse gamma-ray intensity maps from the Energetic Gamma-Ray Experiment Telescope (EGRET) using spherical-wavelet approaches. Correlations at the 99.7% significance level have been detected, at scales around 15° in the WMAP foreground-cleaned W-band and Q-band maps, based on data from regions that are outside the most conservative WMAP foreground mask; no significant correlation is found with the Tegmark cleaned map. The detected correlation is most likely of Galactic origin and thus can help probe the origins of possible Galactic foreground residuals and ultimately remove them from measured microwave sky maps.

CSL 1 is a peculiar object (R.A. = 12^h23^m305, decl. = -12°38'570 [J2000.0]) that, because of its photometric and spectroscopic properties, is possibly the first case of gravitational lensing by a cosmic string. In this Letter we present additional evidence, based on medium to high resolution VLT+FORS1 observations, that the spectra of the two components of CSL 1 are identical within a confidence level higher than 98% and that the velocity difference of the two components is consistent with zero. This result adds further confidence to the interpretation of the system as a true lens.

We present the first spectroscopic measurement of the spatial cross-correlation function between damped Lyα systems (DLAs) and Lyman break galaxies (LBGs). Analysis of deep u'BVRI imaging of nine QSO fields with 11 known z ~ 3 DLAs has spectroscopically confirmed 211 z > 2 LBGs to R = 25.5. We find strong evidence for an overdensity of LBGs near DLAs versus random, the results of which are similar to that of LBGs near other LBGs. A maximum likelihood cross-correlation analysis finds the best-fit value for the correlation length to be r₀ = 2.9h^-1 Mpc using a fixed value of γ = 1.6. The implications of the DLA-LBG clustering amplitude on the average dark matter halo mass of DLAs are discussed.

We identify a population of luminous compact blue galaxies (LCBGs) in two galaxy clusters: MS 0451.6-0305 (z = 0.54) and Cl 1604+4304 (z = 0.9). LCBGs are identified via photometric characteristics and photometric redshifts derived from broad- and narrowband images taken with the WIYN telescope and the Hubble Space Telescope (HST). We analyze their surface densities and clustering properties to find that they compose a statistically significant portion (42% and 53%, respectively) of the Butcher-Oemler (BO) galaxies in both clusters and that their spatial distributions are best characterized by a shell model. The enhancement of the projected space density of LCBGs with M_B < -18.5 in the clusters relative to the field is 3-10 times higher than the BO population as a whole but 2 times lower than the red population, except in the core where LCBGs are absent. Assuming some fading, a natural descendant would be small, low-luminosity galaxies found preferentially in today's clusters, such as dE galaxies.

We present Spitzer 24 μm imaging of 1.5 < z < 2.5 distant red galaxies (DRGs) in the 10' × 10' extended Hubble Deep Field-South of the Multiwavelength Survey by Yale-Chile. We detect 65% of the DRGs with K_AB < 23.2 mag at S_{24 μm} ≳ 40 μJy and conclude that the bulk of the DRG population is dusty active galaxies. A mid-infrared (MIR) color analysis with IRAC data suggests that the MIR fluxes are not dominated by buried AGNs, and we interpret the high detection rate as evidence for a high average star formation rate of ⟨SFR⟩ = 130 ± 30 M_☉ yr^-1. From this, we infer that DRGs are important contributors to the cosmic star formation rate density at z ~ 2, at a level of ~0.02 M_☉ yr^-1 Mpc^-3 to our completeness limit of K_AB = 22.9 mag.

We measure the evolution of the rest-frame K-band fundamental plane from z = 1 to the present by using IRAC imaging of a sample of early-type galaxies in the Chandra Deep Field-South at z ~ 1 with accurately measured dynamical masses. We find that M/L_K evolves as Δ ln(M/L_K) = (-1.18 ± 0.10)z, which is slower than in the B band [Δ ln(M/L_B) = (-1.46 ± 0.09)z]. In the B band, the evolution has been demonstrated to be strongly mass-dependent. In the K band, we find a weaker trend: galaxies more massive than M = 2 × 10¹¹M_☉ evolve as Δ ln(M/L_K) = (-1.01 ± 0.16)z; less massive galaxies evolve as Δ ln(M/L_K) = (-1.27 ± 0.11)z. As expected from stellar population models, the evolution in M/L_K is slower than the evolution in M/L_B. However, when we make a quantitative comparison, we find that the single-burst Bruzual-Charlot models do not fit the results well, unless large dust opacities are allowed at z = 1. Models with a flat IMF fit better; Maraston models with a different treatment of AGB stars fit best. These results show that the interpretation of rest-frame near-IR photometry is severely hampered by model uncertainties and therefore that the determination of galaxy masses from rest-frame near-IR photometry may be harder than was thought before.

Polar ring galaxies are flattened stellar systems with an extended ring of gas and stars rotating in a plane almost perpendicular to the central galaxy. We show that their formation can occur naturally in a hierarchical universe where most low-mass galaxies are assembled through the accretion of cold gas infalling along megaparsec-scale filamentary structures. Within a large cosmological hydrodynamical simulation, we find a system that closely resembles the classic polar ring galaxy NGC 4650A. How galaxies acquire their gas is a major uncertainty in models of galaxy formation, and recent theoretical work has argued that cold accretion plays a major role. This idea is supported by our numerical simulations and the fact that polar ring galaxies are typically low-mass systems.

Early-time X-ray observations of gamma-ray bursts (GRBs) with the Swift satellite have revealed a more complicated phenomenology than was known before. In particular, the presence of flaring activity on a wide range of timescales probably requires late-time energy production within the GRB engine. Since the flaring activity is observed in both long and short GRBs, its origin must be within what is in common for the two likely progenitors of the two classes of bursts: a hyperaccreting accretion disk around a black hole of a few solar masses. Here we show that some of the observational properties of the flares, such as the duration-timescale correlation, and the duration-peak luminosity anticorrelation displayed by most flares within a given burst, are qualitatively consistent with viscous disk evolution, provided that the disk at large radii either fragments or otherwise suffers large-amplitude variability. We discuss the physical conditions in the outer parts of the disk and conclude that gravitational instability, possibly followed by fragmentation, is the most likely candidate for this variability.

In recent years, near-IR and X-ray flares have been detected from the Galaxy's central radio point source, Sagittarius A* (Sgr A*), believed to be an ~3 × 10⁶M_☉ supermassive black hole. In some cases, the transient emission appears to be modulated with a (quasi-)periodic oscillation (QPO) of ~17-20 minutes. The implied ~3r_S size of the emitter (where r_S ≡ 2GM/c² is the Schwarzschild radius) points to an instability—possibly induced by accretion—near the marginally stable orbit (MSO) of a slowly spinning object. But Sgr A* is not accreting via a large, "standard" disk; instead, the low-density environment surrounding it apparently feeds the black hole with low angular momentum clumps of plasma that circularize within ~(10-300)r_S and merge onto a compact, hot disk. In this Letter, we follow the evolution of the disk following such an event, and we show that a Rossby wave instability, particularly in its magnetohydrodynamic (MHD) form, grows rapidly and produces a period of enhanced accretion lasting several hours. Both the amplitude of this response and its duration match the observed flare characteristics rather well.

We present high-precision photometry of the hypervelocity star SDSS J090745.0+024507 (hereafter HVS), which has a Galactic rest-frame radial velocity of v = 709 km s^-1, and thus has likely been ejected from the supermassive black hole in the Galactic center. Our data were obtained on two nights using the MMT 6.5 m telescope and is supplemented by lower precision photometry obtained on four nights using the FLWO 1.2 m telescope. The high-precision photometry indicates that the HVS is a low-amplitude variable. Assuming a periodic, sinusoidal model for this variation, we obtain period P = 0.2-2 days and amplitude A = 2%-10%. Together with the known effective temperature of T_eff ≃ 10,500 K (spectral type B9), this variability implies that the HVS is probably a MS star, since BHB stars do not appear to be variable. Thus, we resolve the previously reported twofold degeneracy in the luminosity and distance of the star. The variability, effective temperature, and possible periodicity of the HVS indicate that it likely belongs to the class of slowly pulsating B-type main-sequence stars. The HVS has a heliocentric distance of 71 kpc and an age of ≲0.35 Gyr. The time of ejection from the center of the Galaxy is ≤100 Myr, and thus the existence of the HVS constitutes observational evidence of a population of young stars in the proximity of the central supermassive black hole ~0.1 Gyr ago. It is possible that the HVS was a member of a binary that was tidally disrupted by the central black hole; we discuss constraints on the properties of the companion's orbit.

We report the discovery of an X-ray pulsar in the young, massive Galactic star cluster Westerlund 1. We detected a coherent signal from the brightest X-ray source in the cluster, CXO J164710.2-455216, during two Chandra observations on 2005 May 22 and June 18. The period of the pulsar is 10.6107(1) s. We place an upper limit to the period derivative of < 2 × 10^-10 s s^-1, which implies that the spin-down luminosity is ≤ 3 × 10³³ ergs s^-1. The X-ray luminosity of the pulsar is L_X ≈ 3 × 10³³(D/5 kpc)² ergs s^-1, and the spectrum can be described by a kT = 0.61 keV blackbody with a radius of R_bb = 0.27 ± 0.03(D/5 kpc) km. Deep infrared observations reveal no counterpart with K < 18.5, which rules out a binary companion with M > 1 M_☉. Taken together, the properties of the pulsar indicate that it is a magnetar. The rarity of slow X-ray pulsars and the position of CXO J164710.2-455216 only 16 from the core of Westerlund 1 indicates that it is a member of the cluster with >99.97% confidence. Westerlund 1 contains 07 V stars with initial masses M_i ≈ 35 M_☉ and >50 post-main-sequence stars that indicate the cluster is 4 ± 1 Myr old. Therefore, the progenitor to this pulsar had an initial mass M_i > 40 M_☉. This is the most secure result among a handful of observational limits to the masses of the progenitors to neutron stars.

We present new Spitzer Space Telescope observations of the young cluster NGC 2264. Observations at 24 μm with the Multiband Imaging Photometer have enabled us to identify the most highly embedded and youngest objects in NGC 2264. This Letter reports on one particular region of NGC 2264 where bright 24 μm sources are spatially configured in curious linear structures with quasi-uniform separations. The majority of these sources (~60%) are found to be protostellar in nature, with Class I spectral energy distributions. Comparison of their spatial distribution with submillimeter data from Wolf-Chase et al. and millimeter data from Peretto et al. shows a close correlation between the dust filaments and the linear spatial configurations of the protostars, indicating that star formation is occurring primarily within dense, dusty filaments. Finally, the quasi-uniform separations of the protostars are found to be comparable in magnitude to the expected Jeans length, suggesting thermal fragmentation of the dense filamentary material.

The observations from 1998 April 20 taken with the Coronal Diagnostics Spectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO) of a coronal loop on the limb have shown that the plasma was multithermal along each line of sight investigated, both before and after background subtraction. The latter result relied on emission measure (EM) loci plots, but in this Letter, we used a forward-folding technique to produce differential emission measure (DEM) curves. We also calculate DEM-weighted temperatures for the chosen pixels and find a gradient in temperature along the loop as a function of height that is not compatible with the flat profiles reported by numerous authors for loops observed with the EUV Imaging Telescope (EIT) on SOHO and the Transition Region and Coronal Explorer (TRACE). We also find discrepancies in excess of the mathematical expectation between some of the observed and predicted CDS line intensities. We demonstrate that these differences result from well-known limitations in our knowledge of the atomic data and are to be expected. We further show that the precision of the DEM is limited by the intrinsic width of the ion emissivity functions that are used to calculate the DEM, which for the EUV lines considered is of the order d log T = 0.2-0.3. Hence, we conclude that peaks and valleys in the DEM, while in principle not impossible, cannot be confirmed from the data.

Resonance scattering has often been invoked to explain the disagreement between the observed and predicted line ratios of Fe XVII λ15.01 to Fe XVII λ15.26 (the "3C/3D" ratio). In this process photons of λ15.01, with its much higher oscillator strength, are preferentially scattered out of the line of sight, thus reducing the observed line ratio. Recent laboratory measurements, however, have found significant inner-shell Fe XVI lines at 15.21 and 15.26 Å, suggesting that the observed 3C/3D ratio results from blending. Given our new understanding of the fundamental spectroscopy, we have reexamined the original solar spectra, identifying the Fe XVI λ15.21 line and measuring its flux to account for the contribution of Fe XVI to the λ15.26 flux. Deblending brings the 3C/3D ratio into good agreement with the experimental ratio; hence, we find no need to invoke resonance scattering. Low opacity in Fe XVII λ15.01 also implies low opacity for Fe XV λ284.2, ruling out resonance scattering as the cause of the fuzziness of TRACE and SOHO-EIT 284 Å images. The images must, instead, be unresolved due to the large number of structures at this temperature. Insignificant resonance scattering implies that future instruments with higher spatial resolution could resolve the active region plasma into its component loop structures.

We observed a transequatorial loop (TEL) connecting NOAA Active Regions 10652 and 10653 at the west solar limb on 2004 July 29 with the Extreme-Ultraviolet Imaging Telescope (EIT) and the Coronal Diagnostic Spectrometer (CDS) aboard the Solar and Heliospheric Observatory. Only the loop's northern leg was observed with CDS. The loop appeared bright and cospatial in extreme-ultraviolet emission lines from ions formed over a wide range of temperature (T, in kelvins), including He I (log T = 4.0), O III (log T = 4.9), O IV (5.2), O V (5.4), Ne VI (5.6), Ca X (5.9), Mg X (6.1), and Fe XII (log T = 6.1). This indicates that the loop plasma was multithermal and covered roughly 2 orders of magnitude in temperature. Our measurement of He I, O III, and O IV line emission reveals the coolest plasma ever detected in a TEL. The most likely explanation for the wide range of cospatial temperatures in the TEL is that it consisted of numerous sub-resolution strands, all at different temperatures. Each of the lines that are formed at temperatures less than 10⁶ K exhibited relative Doppler blueshifts in the TEL that correspond to velocities toward the observer larger than 30 km s^-1, where the two strongest cool lines (He I at 584.3 Å and O V at 629.7 Å) yielded maximum values of 37 and 41 km s^-1, respectively. The presence of cool plasma in the TEL at heights several times those of the cool ions' scale heights suggests that the loop remained visible at low temperatures by maintaining a steady flow of cool plasma.