Table of contents

The sizes and shapes of voids in a galaxy survey depend not only on the physics of structure formation, but also on the sampling density of the survey and on the algorithm used to define voids. Using an N-body simulation with a τCDM power spectrum, we study the properties of voids in samples with different number densities of galaxies, in both redshift space and real space. When voids are defined as totally empty regions of space, their characteristic volume is strongly dependent on sampling density; when they are defined as regions whose density is 0.2 times the mean galaxy density, the dependence is less strong. We compare two void-finding algorithms, one in which voids are nonoverlapping spheres, and one, based on the algorithm of Aikio & Mähönen, that does not predefine the shape of a void. Regardless of the algorithm chosen, the characteristic void size is larger in redshift space than in real space, and is larger for low sampling densities than for high sampling densities. We define an elongation statistic Q that measures the tendency of voids to be stretched or squashed along the line of sight. Using this statistic, we find that at sufficiently high sampling densities (comparable to the number density of L > L_* galaxies), large voids tend to be slightly elongated along the line of sight in redshift space.

We present quantitative investigations of the weak-lensing effect on the two-point correlation functions of local maxima (hot spots), ξ_pk-pk(θ), in cosmic microwave background (CMB) maps. The lensing effect depends on the projected mass fluctuations between today and redshift z_rec ≈ 1100. If we adopt the Gaussian assumption for the primordial temperature fluctuations field, the peak statistics can provide additional information about the intrinsic distribution of hot spots: that those pairs have some characteristic separation angles. The weak lensing then redistributes hot spots in the observed CMB maps from the intrinsic distribution and consequently imprints non-Gaussian signatures onto ξ_pk-pk(θ). In particular, since the intrinsic ξ_pk-pk(θ) has a pronounced depression feature around the angular scale of θ ≈ 70' for a flat universe, the weak lensing induces a large smoothing at that scale. We show that the lensing signature therefore has an advantage for effectively probing mass fluctuations with large wavelength modes around λ ≈ 50 h^-1 Mpc. To reveal the detectability, we performed numerical experiments with specifications of the Microwave Anisotropy Probe (MAP) and Planck Surveyor, including the instrumental effects of beam smoothing and detector noise. We find that our method can successfully provide constraints on the amplitude of the mass fluctuations and cosmological parameters in a flat universe with and without the cosmological constant, provided that we use maps with the 65% sky coverage expected from Planck.

Thermal and chemical evolution of primordial gas clouds irradiated with far-ultraviolet (FUV; hν < 13.6 eV) radiation is investigated. In clouds irradiated by intense FUV radiation, sufficient hydrogen molecules to be important for cooling are never formed. However, even without molecular hydrogen, if the clouds are massive enough, they start collapsing via atomic hydrogen line cooling. Such clouds continue to collapse almost isothermally owing to successive cooling by H^- free-bound emission up to the number density of 10¹⁶ cm^-3. Inside the clouds, the Jeans mass eventually falls well below a solar mass. This indicates that hydrogen molecules are dispensable for low-mass primordial star formation, provided that fragmentation of the clouds occurs at sufficiently high density.

We compute the bispectrum for the galaxy distribution in the IRAS QDOT, 2 Jy, and 1.2 Jy redshift catalogs for wavenumbers 0.05 ≤ k ≤ 0.2 h Mpc^-1 and compare the results with predictions from gravitational instability in perturbation theory. Taking into account redshift-space distortions, nonlinear evolution, the survey selection function, and discreteness and finite-volume effects, all three catalogs show evidence for the dependence of the bispectrum on the configuration shape predicted by gravitational instability. Assuming Gaussian initial conditions and local biasing parameterized by linear and nonlinear bias parameters b₁ and b₂, a likelihood analysis yields 1/b₁ = 1.32, 1.15 and b₂/b = -0.57, -0.50 for the 2 and 1.2 Jy samples, respectively. This implies that IRAS galaxies trace dark matter increasingly weakly as the density contrast increases, consistent with their being underrepresented in clusters. In a model with χ² non-Gaussian initial conditions, the bispectrum displays an amplitude and scale dependence different from that found in the Gaussian case; if IRAS galaxies do not have bias b₁ > 1 at large scales, χ² non-Gaussian initial conditions are ruled out at the 95% confidence level. The IRAS data do not distinguish between Lagrangian and Eulerian local bias.

We report the detection of significant Lyman-continuum flux in the composite spectrum of 29 Lyman-break galaxies (LBGs) with redshifts ⟨z⟩ = 3.40 ± 0.09. After correction for opacity due to intervening absorption using a new composite QSO spectrum evaluated at the same redshift, the ratio of emergent flux density at 1500 Å in the rest frame to that in the Lyman continuum is L(1500 )/L(900 ) = 4.6 ± 1.0. If the relative intensity of the inferred escaping Lyman-continuum radiation is typical of LBGs at z ~ 3 (the galaxies in this sample are drawn from the bluest quartile of LBG spectral energy distributions due to known selection effects), then observed LBGs produce about 5 times more H-ionizing photons per unit comoving volume than QSOs at z ~ 3. The associated contribution to the metagalactic ionizing radiation field is J_ν(912 Å) ≈ 1.2 ± 0.3 × 10^-21 ergs s^-1 cm^-2 Hz^-1 sr^-1 at z ~ 3, very close to most estimates of the radiation field background based on the "proximity effect." A preliminary analysis of the density of faint QSOs in our Lyman-break galaxy survey indicates that the standard extrapolated QSO luminosity function may slightly overpredict the QSO contribution to J_ν(912 ) at z ~ 3. We briefly discuss the implications of a galaxy-dominated UV background at high redshifts.

We present time-resolved optical spectroscopy of the afterglows of the gamma-ray bursts GRB 990510 and GRB 990712. Through the identification of several absorption lines in the first-epoch GRB 990510 spectrum, we determine the redshift for this burst at z ≥ 1.619. No clear emission lines are detected. The strength of the Mg I feature is indicative of a dense environment, most likely the host galaxy of GRB 990510. Although the host is extremely faint (V ≳ 28), the GRB afterglow allows us to probe its interstellar medium and—in principle—to measure its metallicity. The optical spectrum of GRB 990712 (whose host galaxy is the brightest of the known GRB hosts at cosmological redshifts) shows clear features both in emission and absorption, at a redshift of z = 0.4331 ± 0.0004. On the basis of several line emission diagnostic diagrams, we conclude that the host galaxy of GRB 990712 is most likely an H II galaxy. We derive an unreddened [O II] star formation rate of 2.7 ± 0.8 M_☉ yr^-1. Correcting for the measured extinction intrinsic to the host galaxy (A_V = 3.4), this value increases to 35M_☉ yr^-1. The [O II] equivalent width, compared to that of field galaxies at z ≤ 1, also suggests that the host of GRB 990712 is vigorously forming stars. We employ the oxygen and Hβ emission-line intensities to estimate the global oxygen abundance for the host of GRB 990712: log(O/H) = -3.7 ± 0.4, which is slightly below the lowest metallicity one finds in nearby spiral galaxies. For both GRBs we study the time evolution of the absorption lines, whose equivalent width might be expected to change with time if the burst resides in a dense compact medium. We find no evidence for a significant change in the Mg II width.

We report data for I-band surface brightness fluctuation (SBF) magnitudes, (V-I) colors, and distance moduli for 300 galaxies. The survey contains E, S0, and early-type spiral galaxies in the proportions of 49 : 42 : 9 and is essentially complete for E galaxies to Hubble velocities of 2000 km s^-1, with a substantial sampling of E galaxies out to 4000 km s^-1. The median error in distance modulus is 0.22 mag. We also present two new results from the survey. (1) We compare the mean peculiar flow velocity (bulk flow) implied by our distances with predictions of typical cold dark matter transfer functions as a function of scale, and we find very good agreement with cold, dark matter cosmologies if the transfer function scale parameter Γ and the power spectrum normalization σ₈ are related by σ₈Γ^-0.5 ≈ 2 ± 0.5. Derived directly from velocities, this result is independent of the distribution of galaxies or models for biasing. This modest bulk flow contradicts reports of large-scale, large-amplitude flows in the ~200 Mpc diameter volume surrounding our survey volume. (2) We present a distance-independent measure of absolute galaxy luminosity, and show how it correlates with galaxy properties such as color and velocity dispersion, demonstrating its utility for measuring galaxy distances through large and unknown extinction.

We present the results of an extensive analysis of the star formation rates determined from the NICMOS deep images of the northern Hubble Deep Field. We use SED template fitting photometric techniques to determine both the redshift and the extinction for each galaxy in our field. Measurement of the individual extinctions provides a correction for star formation hidden by dust obscuration. We determine star formation rates for each galaxy based on the 1500 Å UV flux and add the rates in redshift bins of width 1.0 centered on integer redshift values. We find a rise in the star formation rate from a redshift of 1 to 2 then a falloff from a redshift of 2 to 3. However, within the formal limits of the error bars this could also be interpreted as a constant star formation rate from a redshift of 1 to 3. The star formation rate from a redshift of 3 to 5 is roughly constant followed by a possible drop in the rate at a redshift of 6. The measured star formation rate at a redshift of 6 is approximately equal to the present day star formation rate determined in other work. The high star formation rate measured at a redshift of 2 is due to the presence of two possible ULIRGs in the field. If real, this represents a much higher density of ULIRGs than measured locally. We also develop a new method to correct for faint galaxies or faint parts of galaxies missed by our sensitivity limit, based on the assumption that the star formation intensity distribution function is independent of redshift. We measure the 1.6 μm surface brightness due to discrete sources and predict the 850 μm brightness of all of our galaxies based on the determined extinction. We find that the far-infrared fluxes predicted in this manner are consistent with the lack of detections of 850 μm sources in the deep NICMOS HDF, the measured 850 μm sky brightness due to discrete sources and the ratio of optical-UV sky brightness to far-infrared sky brightness. From this we infer that we are observing a population of sources that contributes significantly to the total star formation rate and these sources are not overwhelmed by the contribution from sources such as the extremely superluminous galaxies represented by the SCUBA detections. We have estimated the errors in the star formation rate due to a variety of sources including photometric errors, the near-degeneracy between reddening and intrinsic spectral energy distribution as well as the effects of sampling errors and large-scale structure. We have tried throughout to give as realistic and conservative an estimate of the errors in our analysis as possible.

We have performed Monte Carlo simulations of Type Ia supernova (SN Ia) surveys to quantify their efficiency in discovering peculiar overluminous and underluminous SNe Ia. We determined how the type of survey (magnitude-limited, distance-limited, or a hybrid) and its characteristics (observation frequency and detection limit) affect the discovery of peculiar SNe Ia. We find that there are strong biases against the discovery of peculiar SNe Ia introduced by at least four observational effects: the Malmquist bias, the age of the SN Ia at the time of its discovery, the shape of its light curve, and its degree of extinction. Surveys with low observation frequency (less than once per 10 days) tend to discover SNe Ia that are too old for observers to easily recognize their peculiarity. Subluminous SNe Ia are underrepresented in magnitude-limited surveys because they can only be found within a small volume and they remain above the detection limit for less time. Conversely, overluminous SNe Ia are more easily found in magnitude-limited surveys, although their likely association with dusty regions reduces the volume in which they can be discovered. The unbiased rate of peculiar SNe Ia can be recovered only in distance-limited surveys with high observation frequencies and with detection limits that are fainter than the peak magnitude of a subluminous SN Ia in the farthest potential host.

We have compiled a sample of 45 Type Ia supernovae (SNe Ia) discovered by the Lick Observatory Supernova Search (LOSS) and the Beijing Astronomical Observatory Supernova Survey (BAOSS), and determined the rate of spectroscopically peculiar SNe Ia (i.e., SN 1986G-like, SN 1991bg-like, and SN 1991T-like objects) and the luminosity function of SNe Ia. Because of the nature of the two surveys (distance-limited with small baselines and deep limiting magnitudes), nearly all SNe Ia have been discovered in the sample galaxies of LOSS and BAOSS; thus, the observed peculiarity rate and luminosity function of SNe Ia are intrinsic. We find that 36% ± 9% of nearby SNe Ia are peculiar; specifically, the luminosity function of SNe Ia consists of 20% SN 1991T-like, 64% normal, and 16% SN 1991bg-like objects. We have compared our results to those found by earlier studies, and to those found at high redshift. The apparent dearth of SN 1991T-like objects at high redshift may be due to extinction, and especially to the difficulty of recognizing them from spectra obtained past maximum brightness or from spectra with low signal-to-noise ratios. Implications of the high peculiarity rate for the progenitor systems of SNe Ia are also briefly discussed.

It is known that the spectral energy distribution (SED) of the nuclear radiation of narrow-line Seyfert 1 galaxies (NLS1's) has different shapes with respect to that of ordinary broad-line Seyfert 1 galaxies (BLS1's) particularly in X-ray wavelengths. This may cause some differences in the ionization degree and the temperature of gas in narrow-line regions (NLRs) between NLS1's and BLS1's This paper aims to examine whether or not there are such differences in the physical conditions of NLR gas between them. For this purpose, we have compiled the emission-line ratios of 36 NLS1's and 83 BLS1's from the literature. Comparing these two samples, we have found that the line ratios of [O I] λ6300/[O III] λ5007 and [O III] λ4363/[O III] λ5007, which represent the ionization degree and the gas temperature respectively, are statistically indistinguishable between NLS1's and BLS1's. Based on new photoionization model calculations, we show that these results are not inconsistent with the difference of the SED between them. The influence of the difference of SEDs on the highly ionized emission lines is also briefly discussed.

We report the detection of a narrow Fe Kα emission line in the Seyfert 1 galaxy NGC 5548 with the Chandra High-Energy Transmission Grating. In the galaxy frame we measure a center energy of 6.402 keV, a FWHM of 4515 km s^-1, an intensity of 3.6 × 10^-5 photons cm^-2 s^-1, with an equivalent width of 133 eV (errors are 90% confidence for one parameter). The line is only marginally resolved at the 90% confidence level. The line energy is consistent with an origin in cold, neutral matter, but ionization states up to ~Fe XVIII are not ruled out. We cannot constrain the detailed dynamics but, assuming Keplerian motion, the velocity width is consistent with the line being produced in the outer optical/UV broad-line region (BLR) at about a light month from the central X-ray source. We cannot rule out some contribution to the narrow Fe Kα line from a putative, parsec-scale, obscuring torus that is postulated to be a key component of active galactic nuclei (AGNs) unification models. The continuum intensity during the Chandra observation was a factor ~2 less than typical historical levels. If the X-ray continuum was at least a factor of 2 higher in the recent past before the Chandra observation and the narrow Fe Kα intensity had not yet responded to such a change, then the predicted line intensity and equivalent width for an origin in the BLR is within the 90% measurement errors. Anisotropic X-ray continuum illumination of the BLR and/or additional line emission from a torus structure would improve the agreement with observation. Two out of three archival ASCA data sets are consistent with the narrow line being present with the same intensity as in the Chandra observation. However, there is strong evidence that the narrow-line intensity varied and was unusually low during one of the ASCA campaigns. In general, inclusion of the narrow line to model the overall broad Fe Kα line profile in terms of a rotating disk plus black-hole model can have a non-negligible effect on the disk line intensity and variability properties. Variability of the broad disk line in NGC 5548 is difficult to reconcile with the expectations of the simple disk model, even when the narrow-line component is accounted for. It will be critical to ascertain the importance of a similar nondisk Fe Kα line in other Seyfert 1 galaxies. Future monitoring of the narrow Fe Kα component with large collecting area and high spectral resolution will enable reverberation mapping the BLR region, complementary to similar studies using the optical/UV lines, and therefore provide independent constraints on the black-hole mass.

We report the discovery of a new two-image gravitational lens candidate. The system RX J0921+4529 contains two z_s = 1.66 quasars separated by 693 with an H-band magnitude difference of Δm = 1.39. The HST NIC2 H-band images reveal an H = 18.2 spiral galaxy between the quasar images, which is probably a member of a z_l = 0.32 X-ray cluster centered on the field. We detect an extended source near the fainter quasar image but not in the brighter image. If this extended source is the host galaxy of the fainter quasar, then the system is a binary quasar rather than a gravitational lens. VLA observations at 3.6 cm reveal emission from the lens galaxy at the flux level of 1 mJy and a marginal detection of the brighter quasar.

We present a very high signal-to-noise ratio composite spectrum created using 657 radio-selected quasars from the FIRST Bright Quasar Survey. The spectrum spans rest-frame wavelengths 900-7500 Å. Additionally, we present composite spectra formed from subsets of the total data set in order to investigate the spectral dependence on radio loudness and the presence of broad absorption lines. In particular, radio-loud quasars are red compared to radio-quiet quasars, and quasars showing low-ionization broad absorption lines are red compared to other quasars. We compare our composites with those from the Large Bright Quasar Survey. Composite quasar spectra have proved to be valuable tools for a host of applications, and in that spirit we make these publicly available via the FIRST survey web page.

We have obtained Near-Infrared Camera and Multi-Object Spectrometer images of 16 radio-quiet quasars observed as part of a project to investigate the "luminosity/host-mass limit." The limit results were presented in a paper by McLeod, Rieke, & Storrie-Lombardi. In this paper, we present the images themselves, along with one- and two-dimensional analyses of the host galaxy properties. We find that our model-independent one-dimensional technique is reliable for use on ground-based data at low redshifts; that many radio-quiet quasars live in de Vaucouleurs-law hosts, although some of the techniques used to determine host type are questionable; that complex structure is found in many of the hosts, but that there are some hosts that are very smooth and symmetric; and that the nuclei radiate at ~2%-20% of the Eddington rate based on the assumption that all galaxies have central black holes with a constant mass fraction of 0.6%. Despite targeting hard-to-resolve hosts, we have failed to find any that imply super-Eddington accretion rates.

Recent ROSAT studies have identified a significant population of active galactic nuclei (AGNs) that are notably faint in soft X-rays relative to their optical fluxes. Are these AGNs intrinsically X-ray weak or are they highly absorbed? Brandt, Laor, & Wills have systematically examined the optical and UV spectral properties of a well-defined sample of these soft X-ray-weak (SXW) AGNs drawn from the Boroson & Green sample of all the Palomar Green AGNs with z < 0.5. We present ASCA observations of three of these SXW AGNs: PG 1011-040, PG 1535+547 (Mrk 486), and PG 2112+059. In general, our ASCA observations support the intrinsic absorption scenario for explaining soft X-ray weakness; both PG 1535+547 and PG 2112+059 show significant column densities (N_H ≈ 10²²-10²³ cm^-2) of absorbing gas. Interestingly, PG 1011-040 shows no spectral evidence for X-ray absorption. The weak X-ray emission may result from very strong absorption of a partially covered source, or this AGN may be intrinsically X-ray weak. PG 2112+059 is a broad absorption line (BAL) QSO, and we find it to have the highest X-ray flux known of this class. It shows a typical power-law X-ray continuum above 3 keV; this is the first direct evidence that BAL QSOs indeed have normal X-ray continua underlying their intrinsic absorption. Finally, marked variability between the ROSAT and ASCA observations of PG 1535+547 and PG 2112+059 suggests that the soft X-ray-weak designation may be transient, and multiepoch 0.1-10.0 keV X-ray observations are required to constrain variability of the absorber and continuum.

The discovery of EUV and soft X-ray excess emission in clusters of galaxies (the cluster soft excess phenomenon) challenged the notion of the hot (~10⁷-10⁸ K) gas as the only dominant thermal component of the intracluster medium (ICM). The spatial analysis of ROSAT PSPC keV images presented here reveals compelling evidence for substantial amounts of cold gas (H I) in the ICM of the Coma and Virgo Clusters. This finding bolsters the original interpretation of the soft excess as emission from a "warm" (~10⁶ K) gas and points to the scenario of a multiphase ICM where the hot component coexists with gases at subvirial temperatures (the "warm" and "cold" phases).

With the success of the Chandra and XMM missions and the maturation of gravitational lensing techniques, powerful constraints on the orbital structure of cluster dark matter halos are possible. I show that the X-ray emissivity and mass of a galaxy cluster uniquely specify the anisotropy and velocity dispersion profiles of its dark matter halo. I consider hydrostatic as well as cooling flow scenarios and apply the formalism to the lensing cluster Cl 0024+16 and the cooling flow cluster Abell 2199. In both cases, the model predicts a parameter-free velocity dispersion profile that is consistent with independent optical redshift surveys of the clusters.

The axion is a hypothetical elementary particle and a cold dark matter candidate. It could dominate the potential wells of most astrophysical objects. Axions spontaneously decay into nearly monochromatic microwave photons. We present results from a radio telescope search for these axion decay photons of mass m_a = 298-363 μeV in Local Group dwarf galaxies. We report a limit on the axion-to-two-photon coupling constant g_aγγ > 1.0 × 10^-9 GeV^-1.

Far-ultraviolet (FUV) imagery of the edge-on, Sc/SBd galaxy NGC 4631 reveals very strong FUV emission, resulting from active star formation, uniformly distributed along the galactic midplane. Multiband imagery, H I and H II position-velocity curves, and extinction considerations all imply that the emission is from the outer edges of the visible galaxy. The overall FUV morphology of this edge-on disk system is remarkably similar to those of the "chain galaxies" evident at high redshift, thus suggesting a similar interpretation for at least some of those distant objects. FUV, U, B, and V magnitudes, measured for 48 star-forming regions, along with corresponding Hα and Hβ measurements are used to construct diagnostic color-color diagrams. Although there are significant exceptions, most of the star-forming regions are less massive and older than 30 Doradus. Comparison with the expectations from two star formation models yields ages of 2.7 to 10 Myr for the instantaneous burst (IB) model and star formation cutoff ages of 0 to 9 Myr for the continuous star formation (CSF) model. Interpreted in terms of the IB model the photometry implies a total created mass in the 48 star-forming regions of 2.5 × 10⁷M_☉. When viewed as resulting from constant star formation the photometry implies a star formation rate of 0.33 M_☉ yr^-1. These results are compared to those derived from FIR and radio observations. Corrections for FUV emission reprocessed by interstellar grains are estimated. A large ring, ~3 kpc in diameter, of 14 star-forming regions is concentrically located with an expanding H I shell toward the eastern end of the galaxy. Our observations imply that the shell may have been generated primarily by supernovae arising from 5.3 × 10⁴ OB stars in a massive star-forming region beginning about 20 Myr ago, and that the presently observed FUV bright emission is due to second generation stars.

We report the results of a spectroscopic investigation of a sample of 20 of the brightest type 2 Seyfert nuclei. Our goal is to search for the direct spectroscopic signature of massive stars and thereby probe the role of circumnuclear starbursts in the Seyfert phenomenon. The method used is based on the detection of the higher order Balmer lines and He I lines in absorption and the Wolf-Rayet feature at ~4680 Å in emission. These lines are strong indicators of the presence of young (a few Myr) and intermediate-age (a few 100 Myr) stellar populations. In over half the sample, we have detected He I and/or strong stellar absorption features in the high-order (near-UV) Balmer series together with relatively weak lines from an old stellar population. In three others we detect a broad emission feature near 4680 Å that is most plausibly ascribed to a population of Wolf-Rayet stars (the evolved descendants of the most massive stars). We therefore conclude that the blue and near-UV light of over half of the sample is dominated by young and/or intermediate age stars. The "young" Seyfert 2 galaxies have larger far-IR luminosities, cooler mid/far-IR colors, and smaller [O III]/Hβ flux ratios than the "old" ones. These differences are consistent with a starburst playing a significant energetic role in the former class. We consider the possibility that there may be two distinct subclasses of Seyfert 2 nuclei ("starbursts" and "hidden broadline regions" [BLRs]). However, the fact that hidden BLRs have been found in three of the "young" nuclei argues against this and suggests that nuclear starbursts may be a more general part of the Seyfert phenomenon.

We present our analysis of Hubble Space Telescope/Planetary Camera narrow-bandpass and broadband imagery of the inner 3 kpc region of NGC 1068. Our analysis of F160BW and F547M broadband continuum imagery suggests that roughly 40% of the scattered active galactic nucleus (AGN) continuum emission originates from an unobscured single cloud complex largely free from dust with total number densities typical of diffuse clouds in our own Milky Way. The net emission-line fluxes are extracted from continuum-subtracted narrow-bandpass imagery for Ha + [N ], Hβ, S II λλ6717, 6731, and [O III] λ5007. Although the [O ]/(Hα + [N ]) flux ratio shows a sharp drop-off at distances beyond ~4'' northeast of the nucleus, the [O ]/Hb ratio indicates no such decrease. This implies that the ionization of these species is not strongly influenced by shocks associated with the expanding radio lobes as inferred from a previous study. The sharp drop-off seen in the [O ]/(Ha + [N ]) appears to be due to high interstellar reddening immediately beyond the scattering bright clouds near the nucleus, as further evidenced by the high Ha/Hb ratio in this region. The presence of a faint inner spiral arm interior to the sharply defined star formation ring, possibly driven by an outer-inner Linblad resonance, may provide a means of fueling the central AGN, as recently suggested by Yuan and Kuo. Because IR observations do not support the presence of a true AGN torus in NGC 1068, we present a qualitative model in which the radio ejecta has carved out an ionization cone in the high concentration of dense molecular clouds surrounding the nucleus. This picture also implies that the extended conical emission region to the northeast of the nucleus lies near the galactic plane and is surrounded by lower density ambient gas than that surrounding the highly ionized gas southwest of the nucleus.

We analyzed the ~950 ks of spectroscopic data accumulated by the Extreme Ultraviolet Explorer (EUVE) satellite between 1994 and 1997 for the BL Lacertae object Markarian 421. The EUV spectrum is well detected in the 70-110 Å (112-177 eV) range and can be fitted by a power-law model plus an absorption feature in the ~71-75 Å range. Previous studies of EUV absorption features in Mrk 421 and in the other EUV bright BL Lac object, PKS 2155-304, explain this absorption feature as a superposition of Doppler-smeared absorption lines (mainly L- and M-shell transitions of Mg and Ne) originating in high-velocity gas clouds ionized by the beamed continuum of the associated relativistic jet. We show that, for example, Fe IX L could also be a possibility consistent with the marginal detection of oxygen absorption lines in the X-ray range. However physical models are highly sensitive to the assumptions on the photoionizing continuum and the surrounding gas.

The Extreme Ultraviolet Explorer (EUVE) satellite accumulated ~1,000,000 s of public data between 1994 and 1997 for the BL Lacertae object Markarian 421. This is the second of two papers in which we present the results of spectral and temporal analysis of this EUVE data set. We analyze in the present paper the imaging data by means of power spectrum and structure function techniques, while the spectral analysis is presented in a companion paper. We find for Mrk 421 a power spectrum with slope -2.14 ± 0.28 with a break at ~3 days. This is the first time that a break in the power spectrum of a BL Lacertae object has been found. We also find evidence of nonstationarity for Mrk 421 EUV emission.

During 1997, the BL Lac object Mrk 501 went into an extraordinary state of high X-ray and TeV gamma-ray activity, lasting more than 6 months. In this paper we report on the TeV emission characteristics of the source in the subsequent years of 1998 and 1999 as measured with the stereoscopic Cherenkov telescope system of the High-Energy Gamma-Ray Astronomy (HEGRA; La Palma, Canary Islands). Our observations reveal a 1998-1999 mean emission level at 1 TeV of of the flux of the Crab Nebula, a factor of 10 lower than during the year of 1997. A data set of 122 observation hours with the HEGRA telescope system makes it possible to assess for the first time the Mrk 501 TeV energy spectrum for a mean flux level substantially below that of the Crab Nebula with reasonable statistical accuracy. Excluding the data of a strong flare, we find evidence that the 1998-1999 low-flux spectrum is substantially softer (by 0.44 ± 0.1_stat in spectral index) than the 1997 time-averaged spectrum. The 500 GeV to ≃10 TeV energy spectrum can well be described by a power-law model with exponential cutoff: dN/dE ∝ E^-α exp(-E/E₀), with α = 2.31 ± 0.22_stat and E₀ = 5.1()_stat TeV. Within statistical accuracy, a pure power-law model also gives an acceptable fit to the data: dN/dE ∝ E^-Γ, with Γ = 2.76 ± 0.08_stat. After presenting the 1998-1999 TeV characteristics of the source, we discuss the implications of the results.

In this paper we consider three-integral axisymmetric models for NGC 4649 and NGC 7097, addressing the question of the presence of dark matter in these elliptical galaxies. The data for NGC 7097 can be modeled without a dark matter halo. The central region of NGC 4649 could be hosting a black hole. The kinematical data of NGC 4649 are not inconsistent with a constant mass-to-light ratio model, but a dynamical model with a moderate amount of dark matter better reproduces the observed anisotropy on the major axis. In addition, we look more closely at the issue of the classification of elliptical galaxies. Both galaxies can be placed in different families of elliptical galaxies based on the isophote shape. In this paper we set out to investigate whether the kinematic information can be used to specify a second classification parameter. We propose the use of normalized distribution functions, which are the three-integral distribution functions divided by the two-integral model (constructed from photometry alone), and argue why this is an efficient way of presenting the dynamics of elliptical galaxies. We show that they can be used to characterize the rotational state of a galaxy in a more specific way than merely indicating the amount of rotation or counterrotation. Furthermore, the distribution function for NGC 7097 suggests that the counterrotation is caused by stars spread over large radii in the galaxy, implying that this phenomenon is not related to a compact group of stars. Hence, the origin of the counterrotation should not be found in a recent merger in the first place. This shows that it is important to construct dynamic models with a distribution function. The distribution function is the tool used to visualize structure in phase space, which is the only stellar dynamic remnant of galaxy formation.

The noncircular streaming motions in barred galaxies are sensitive to the mass of the bar and can be used to lift the degeneracy between disk and dark matter halo encountered when fitting axisymmetric rotation curves of disk galaxies. In this paper we present photometric and kinematic observations of NGC 4123, a barred galaxy of modest size (V_rot = 130 km s^-1, L = 0.7L_*), that reveal strong noncircular motions. The bar has straight dust lanes and an inner Lindblad resonance. The disk of NGC 4123 has no sign of truncation out to 10 scale lengths, and star-forming regions are found well outside R₂₅. A Fabry-Perot Hα velocity field shows velocity jumps of greater than 100 km s^-1 at the location of the dust lanes within the bar, indicating shocks in the gas flow. VLA observations yield the velocity field of the H I disk. Axisymmetric mass models yield good fits to the rotation curve outside the bar region for disk I-band M/L of 2.25 or less, and dark halos with either isothermal or power-law profiles can fit the data well. In a companion paper we model the full two-dimensional velocity field, including noncircular motions, to determine the stellar M/L and the mass of the dark halo.

We report a dynamical determination of the separate contributions of disk and dark halo masses to the rotation curve of a spiral galaxy. We use fluid-dynamical models of gas flow in the barred galaxy NGC 4123 to constrain the dynamical properties of the galaxy: disk M/L, bar pattern speed, and the central density and scale radius of the dark halo. We derive a realistic barred potential directly from the light distribution. For each model we assume a value of the stellar M/L and a bar pattern speed Ω_p and add a dark halo to fit the rotation curve. We then compute the gas flow velocities with a two-dimensional gasdynamical code and compare the model flow patterns to a two-dimensional velocity field derived from Fabry-Perot observations. The strong shocks and noncircular motions in the observed gas flow require a high stellar M/L and a fast-rotating bar. Models with I-band disk M/L of 2.0-2.5 h₇₅, or 80%-100% of the maximum disk value, are highly favored. The corotation radius of the bar must be ≤1.5 times the bar semimajor axis. These results contradict some recent claimed "universal" galaxy disk/halo relations, since NGC 4123 is of modest size (rotation curve maximum 145 km s^-1 and V_flat = 130 km s^-1) yet quite disk-dominated. The dark halo of NGC 4123 is less concentrated than favored by current models of dark halos based on cosmological simulations. Since some 30% of bright disk galaxies are strongly barred and have dust lanes indicating shock morphology similar to that of NGC 4123, it is likely that they also have high stellar M/L and low-density halos. We suggest that luminous matter dominates inside the optical radius R₂₅ of high surface brightness disk galaxies.

The modest extinction and reasonably face-on viewing geometry make the luminous infrared galaxy NGC 1614 an ideal laboratory for the study of a powerful starburst. Hubble Space Telescope/near-infrared camera and multiobject spectrometer (NICMOS) observations show (1) deep CO stellar absorption, tracing a starburst nucleus about 45 pc in diameter (2) surrounded by a ~600 pc diameter ring of supergiant H II regions revealed in Paα line emission (3) lying within a molecular ring indicated by its extinction shadow in H-K, (4) all at the center of a disturbed spiral galaxy. The luminosities of the giant H II regions in the ring are extremely high, an order of magnitude brighter than 30 Doradus; very luminous H II regions, comparable with 30 Dor, are also found in the spiral arms of the galaxy. Luminous stellar clusters surround the nucleus and lie in the spiral arms, similar to clusters observed in other infrared luminous and ultraluminous galaxies. The star-forming activity may have been initiated by a merger between a disk galaxy and a companion satellite whose nucleus appears in projection about 300 pc to the northeast of the nucleus of the primary galaxy. The relation of deep stellar CO bands to surrounding ionized gas rings to molecular gas indicates that the luminous starburst started in the nucleus and is propagating outward into the surrounding molecular ring. This hypothesis is supported by evolutionary starburst modeling that shows that the properties of NGC 1614 can be fitted with two short-lived bursts of star formation separated by 5 Myr (and by inference by a variety of models with a similar duration of star formation). The total dynamical mass of the starburst region of 1.3 × 10⁹M_☉ is mostly accounted for by the old prestarburst stellar population. Although our starburst models use a modified Salpeter initial mass function (IMF) (turning over near 1 M_☉), the tight mass budget suggests that the IMF may contain relatively more 10-30 M_☉ stars and fewer low-mass stars than the Salpeter function. The dynamical mass is nearly 4 times smaller than the mass of molecular gas estimated from the standard ratio of ¹²CO (1-0) to H₂. A number of arguments place the mass of gas in the starburst region at ~25% of the dynamical mass, nominally about 1/15 and with an upper limit of 1/10 of the amount estimated from ¹²CO and the standard ratio.

Recent multiwaveband observations of Seyfert nuclei and QSOs established significant deviations in the spectral shape of the big blue bump from a blackbody spectral shape; soft X-ray excess has a spectral index α (F_ν ∝ ν^-α) of 1.6 and hard X-ray tail with α of ~0.7. We construct a disk-corona model which accounts for such broadband spectral properties. We study the emission spectrum emerging from a vertical disk-corona structure composed of two-temperature plasma by solving hydrostatic equilibrium and radiative transfer self-consistently. A fraction f of viscous heating due to mass accretion is assumed to be dissipated in a corona with a Thomson optical depth of τ_c, where advective cooling is also included, and a remaining fraction, 1 - f, dissipates within a main body of the disk. Our model can nicely reproduce the soft X-ray excess with a power-law shape and the hard tail extending to ~50 keV. The different spectral slopes (α ~ 1.5 below 2 keV and ~0.5 above) are the results of different emission mechanisms and different sites; the former slope is due to unsaturated Comptonization from the innermost zone, and the latter is due to a combination of the Comptonization, bremsstrahlung, and a reflection of the coronal radiation at the disk-corona boundary from the inner to surrounding zone (≤300 Schwarzschild radii). The emergent optical spectrum is redder (α ~ 0.3) than that of the standard disk (α ~ -0.3), being consistent with observations, due to the different efficiencies of spectral hardening of disk emission at different radii. Further, we find that the cutoff frequency of the hard X-ray (~coronal electron temperature) and broadband spectral shape are insensitive to the black hole mass, while the peak frequency of the big blue bump is sensitive to the mass as the peak frequency ∝M.

There are two different strategies of follow-up observations for the detection of planets by using microlensing. One is detecting the light-curve anomalies affected by the planetary caustic from continuous monitoring of all events detected by microlensing survey programs (type I strategy), and the other is detecting anomalies near the peak amplification affected by the central caustic from intensive monitoring of high-amplification events (type II strategy). It was shown by Griest & Safizadeh that the type II strategy yields high planet detection efficiency per event. However, it is not known whether the planet detection rate by this strategy can make up a substantial fraction of the total rate. In this paper, we estimate the relative planet detection rates expected under the two follow-up observation strategies. From this estimation, we find that the rate under the type II strategy is substantial and will comprise ~- of the total rate. We also find that compared to the type I strategy the type II strategy is more efficient in detecting planets located outside of the lensing zone. We determine the optimal monitoring frequency of the type II strategy to be ~20 times per night, which can be easily achieved by the current microlensing follow-up programs even with a single telescope.

We use three-dimensional (3D) numerical magnetohydrodynamic simulations to follow the evolution of cold, turbulent, gaseous systems with parameters chosen to represent conditions in giant molecular clouds (GMCs). We present results of three model cloud simulations in which the mean magnetic field strength is varied (B₀ = 1.4-14 μG for GMC parameters), but an identical initial turbulent velocity field is introduced. We describe the energy evolution, showing that (1) turbulence decays rapidly, with the turbulent energy reduced by a factor 2 after 0.4-0.8 flow crossing times (~2-4 Myr for GMC parameters), and (2) the magnetically supercritical cloud models gravitationally collapse after time ≈6 Myr, while the magnetically subcritical cloud does not collapse. We compare density, velocity, and magnetic field structure in three sets of model "snapshots" with matched values of the Mach number ≈ 9,7,5. We show that the distributions of volume density and column density are both approximately log-normal, with mean mass-weighted volume density a factor 3-6 times the unperturbed value, but mean mass-weighted column density only a factor 1.1-1.4 times the unperturbed value. We introduce a spatial binning algorithm to investigate the dependence of kinetic quantities on spatial scale for regions of column density contrast (ROCs) on the plane of the sky. We show that the average velocity dispersion for the distribution of ROCs is only weakly correlated with scale, similar to mean size-line width distributions for clumps within GMCs. We find that ROCs are often superpositions of spatially unconnected regions that cannot easily be separated using velocity information; we argue that the same difficulty may affect observed GMC clumps. We suggest that it may be possible to deduce the mean 3D size-line width relation using the lower envelope of the 2D size-line width distribution. We analyze magnetic field structure and show that in the high-density regime n ≳ 10³ cm^-3, total magnetic field strengths increase with density with logarithmic slope ~1/3-2/3. We find that mean line-of-sight magnetic field strengths may vary widely across a projected cloud and are not positively correlated with column density. We compute simulated interstellar polarization maps at varying observer orientations and determine that the Chandrasekhar-Fermi formula multiplied by a factor ~0.5 yields a good estimate of the plane-of sky magnetic field strength, provided the dispersion in polarization angles is ≲25°.

We present very deep and accurate photometry of the open cluster M35. We have observed this association in the Cousins R, I filters, together with the Johnson V filter. We have covered a region of 27.5 × 27.5 square arcmin, equivalent to a fifth of the total area of the cluster. The data range from I_c = 12.5 to 23.5 mag, and the color intervals are 0.4 ≤ (V-I)_c ≤ 3.0, 0.5 ≤ (R-I)_c ≤ 2.5. Roughly, these values span from 1.6 M_☉ down to the substellar limit, in the case of cluster members. By using the location of the stars on color-magnitude and color-color diagrams, we have selected candidate members of this cluster. We have merged our sample with previously published data and obtained a color-magnitude diagram for the complete stellar population of the cluster, covering the spectral range early B-mid M. Based on the distribution of field and cluster stars in color-magnitude and color-color diagrams, we estimate that two-thirds of these candidates are likely to be true members of M35. These stars approximately double the number of stars identified as candidate members of this cluster (~2700). We provide the photometry and accurate positions of these stars. The deep photometry has allowed us to study the mass segregation within the cluster, the luminosity function, and mass function. We show that in the magnitude range 13 ≤ I_c ≤ 22 there is a reduced mass segregation, in opposition to what happens to higher mass stars, where the mass segregation is stronger. The luminosity function behaves essentially as the one characteristic of the Pleiades, presenting a peak at I_c ~ 19 mag (M_I ~ 9). Combining our photometry with previous data corresponding to more massive stars, we find that the mass function increases monotonically, when plotted in a log-log form, until it reaches ~0.8 M_☉ (α = 2.59). It remains shallower for less massive stars (α = 0.81 for 0.8-0.2 M_☉), whereas a decrease is observed for stars close to the substellar regime. These different behaviors suggest that at least three mechanisms play a role in the formation of stellar and substellar objects. The total mass of the cluster is ~1600 M_☉ in the area covered by this study.

Using idealized models of the accretion disk, we investigate the relativistic effects on the energy deposition rate via neutrino pair annihilation near the rotation axis of a Kerr black hole. Neutrinos are emitted from the accretion disk. The bending of neutrino trajectories and the redshift due to the disk rotation and gravitation are taken into consideration. The Kerr parameter, a, affects not only behavior of the neutrinos but also the inner radius of the accretion disk. When the deposition energy is mainly contributed by the neutrinos coming from the central part, the redshift effect becomes dominant as a becomes large, and the energy deposition rate is reduced compared with that neglecting the relativistic effects. On the other hand, for a small a, the bending effect becomes dominant and makes the energy increase by factor of 2 compared with that which neglects the relativistic effects. For the disk with a temperature gradient, the energy deposition rate for a small inner radius of the accretion disk is smaller than that estimated by neglecting the relativistic effects. The relativistic effects, especially for a large a, play a negative role in avoiding the baryon contamination problem in gamma-ray bursts.

We report on the 0.5-200 keV spectral properties of Cyg X-1 observed at different epochs with the Narrow Field Instruments of the BeppoSAX satellite. The source was in its soft state during the first observation of 1996 June. In the second observation of 1996 September, the source had parameters characteristic to its hard state. A soft X-ray excess, a broad Fe Kα line and Compton reflection are clearly detected in both states. The soft-state broadband continuum is well modeled by a disk blackbody (accounting for the soft excess) and Compton upscattering of the disk photons by a hybrid, thermal/nonthermal plasma, probably forming a corona above the disk (also giving rise to the Compton-reflection component). In the hard state, the primary hard X-ray spectrum can be well modeled by Compton upscattering of a weak blackbody emission by a thermal plasma at a temperature of ~60 keV. The soft excess is then explained by thermal Comptonization of the same blackbody emission by another hot plasma cloud characterized by a low value of its Compton parameter. Finally, we find the characteristic ratio of the bolometric flux in the soft state to that in the hard state to be about 3. This value is much more compatible with theories of state transitions than the previously reported (and likely underestimated) value of 1.5.

High-energy gamma-ray astronomy is now a well-established field, and several sources have been discovered in the region from a few giga-electron volts up to several tera-electron volts. If sources involving hadronic processes exist, the production of photons would be accompanied by neutrinos too. Other possible neutrino sources could be related to the annihilation of weakly interacting, massive particles (WIMPs) at the center of galaxies with black holes. We present the results of a search for pointlike sources using 1100 upward-going muons produced by neutrino interactions in the rock below and inside the Monopole Astrophysics and Cosmic Ray Observatory (MACRO) detector in the underground Gran Sasso Laboratory. These data show no evidence of a possible neutrino pointlike source or of possible correlations between gamma-ray bursts and neutrinos. They have been used to set flux upper limits for candidate pointlike sources which are in the range 10^-14-10^-15 cm^-2 s^-1.

We report evidence for an Fe Kα fluorescence line feature and disk reflection in the very high, high-, and low-state X-ray spectra of the Galactic microquasar XTE J1748-288 during its 1998 June outburst. Spectral analyses are made on data gathered throughout the outburst by the Rossi X-Ray Timing Explorer Proportional Counter Array. Gaussian line, relativistic disk emission line, and ionized disk reflection models are fitted to the data. In the very high state the line profile appears strongly redshifted, consistent with disk emission from the innermost stable orbits around a maximally rotating Kerr black hole. In the high state the line profile is less redshifted and increasingly prominent. The low-state line profile is very strong (~0.5 keV equivalent width) and centered at 6.7 ± 0.10 keV; disk line emission model fits indicate that the inner edge of the disk fluctuates between ~20R_g and ~100R_g in this state. The disk reflection fraction is traced through the outburst; reflection from an ionized disk is preferred in the very high and high states, and reflection from a relatively neutral disk is preferred in the low state. We discuss the implications of our findings for the binary system dynamics and accretion flow geometry in XTE J1748-288.

We have searched for a methane signature in the infrared spectrum of τ Bootis, produced by the planetary companion. The observations comprise 598 low-noise (signal-to-noise ratio ~100), high-resolution (λ/δλ = 4 × 10⁴) spectra near 3044 cm^-1, which we analyze by cross-correlating with a modeled planetary spectrum based on the work of Burrows & Sharp and Sudarsky, Burrows, & Pinto. The 3 σ random noise level of our analysis is ~ 5 × 10^-5 stellar continuum flux units, and the confusion noise limit—measuring the resemblance of a cross-correlation feature to the spectrum of methane—is ~ 2.5 × 10^-4. We find a significant cross-correlation amplitude of ~ 3.3 × 10^-4 continuum units at a velocity near that of the star. This is likely due to methane from a low-mass companion in a long-period orbit. Fischer, Butler, & Marcy report a long-term velocity drift indicative of such a companion. But the system is known to be a visual binary with an eccentric orbit and is rapidly approaching periastron. Whether the visual companion can account for our observations and the Fischer et al. velocity drift depends on knowing the orbit more precisely. The stability of planetary orbits in this system also depends crucially on the properties of the binary orbit. A second cross-correlation feature, weaker and much more diffuse, has intensity amplitude ~ 2 × 10^-4 continuum units and occurs at a velocity amplitude of 71(±10) km s^-1, in agreement with the orbit claimed for the planet by Cameron et al. Like the first feature, it has passed several tests designed to reject systematic errors. We discuss the possibility that this second feature is due to the planet.

We report the detection of a series of X-ray dips in the Galactic black hole candidate GRS 1915+105 during 1999 June 6-17 from observations carried out with the pointed proportional counters of the Indian X-ray Astronomy Experiment on board the Indian Remote Sensing Satellite (IRS-P3). The observations were made after the source made a transition from a steady low-hard state to a chaotic state, which occurred within a few hours. Dips of about 20-160 s in duration were observed on most of the days. The X-ray emission outside the dips shows a quasi-periodic oscillation (QPO) at ~4 Hz that has characteristics similar to the ubiquitous 0.5-10 Hz QPO seen during the low-hard state of the source. During the onset of dips this QPO is absent, and, also, the energy spectrum is soft and the variability is low compared to the nondip periods. These features gradually reappear as the dip recovers. The onset of the occurrence of a large number of such dips followed the start of a huge radio flare of strength 0.48 Jy (at 2.25 GHz). We interpret these dips as the cause for mass ejection due to the evacuation of matter from an accretion disk around the black hole. We propose that a superposition of a large number of such dip events produces a huge radio jet in GRS 1915+105.

We report optical monitoring of the soft X-ray transient XTE J1550-564 during the 1999 season (1999 January 4-August 24). The first optical observations available in 1999 show that the peak "reflare" brightness had exceeded the peak brightness of the initial optical flare in 1998 September by over half a magnitude. We compare the optical reflare light curves with the total X-ray flux, the power-law flux, and disk flux light curves constructed from the spectral fits to RXTE/PCA data. During the first 60 days of the observed optical reflare, we find no correspondence between the thermal component of the X-rays often associated with a disk and the optical flux—the former remains essentially flat, whereas the latter declines exponentially and exhibits three substantial dips. However, the power-law flux is anticorrelated with the optical dips, suggesting that the optical flux may be up scattered into the X-ray by the hot corona. Periodic modulations were discovered during the final stage of the outburst (May-June), with P = 1.546 ± 0.038 days, and during quiescence (July and August), with P = 1.540 ± 0.008 days. The analysis of the combined data set reveals a strong signal for a unique period at P = 1.541 ± 0.009 days, which we believe to be the orbital period.

The discovery of high-amplitude brightness oscillations during type I X-ray bursts from six low-mass X-ray binaries has provided a powerful new tool to study the properties of matter at supranuclear densities, the effects of strong gravity, and the propagation of thermonuclear burning. There is substantial evidence that these brightness oscillations are produced by spin modulation of one or two localized hot spots confined to the stellar surface. It is therefore important to calculate the expected light curves produced by such hot spots under various physical assumptions, so that comparison with the observed light curves may most sensitively yield information about the underlying physical quantities. In this paper we make general relativistic calculations of the light curves and oscillation amplitudes produced by a rotating neutron star with one or two hot spots as a function of spot size, stellar compactness, rotational velocity at the stellar surface, spot location, orientation of the line of sight of the observer, and the angular dependence of the surface specific intensity. For the case of two emitting spots we also investigate the effects of having spot separations less than 180° and the effects of having asymmetries in the brightness of the two spots. We find that stellar rotation and beaming of the emission tend to increase the observed oscillation amplitudes whereas greater compactness and larger spot size tend to decrease them. We also show that when two emitting spots are either nonantipodal or asymmetric in brightness, significant power at the first harmonic is generated. By applying these results to 4U 1636-536, the two emitting spots of which produce power at the first harmonic, we place strong constraints on the neutron star's magnetic field geometry. We also show that the data on the phase lags between photons of different energies in the persistent pulsations in SAX J1808-58 can be fitted well with a model in which the observed hard leads are due to Doppler beaming.

We study the timing properties of the bursting atoll source 4U 1728-34 as a function of its position in the X-ray color-color diagram. In the island part of the color-color diagram (corresponding to the hardest energy spectra), the power spectrum of 4U 1728-34 shows several features such as a band-limited noise component present up to a few tens of Hz, a low-frequency quasi-periodic oscillation (LFQPO) at frequencies between 20 and 40 Hz, a peaked noise component around 100 Hz, and one or two QPOs at kHz frequencies. In addition to these, in the lower banana (corresponding to softer energy spectra) we also find a very low frequency noise (VLFN) component below ~1 Hz. In the upper banana (corresponding to the softest energy spectra), the power spectra are dominated by the VLFN, with a peaked noise component around 20 Hz. We find that the frequencies of the kHz QPOs are well correlated with the position in the X-ray color-color diagram. For the frequency of the LFQPO and the break frequency of the broadband noise component, the relation appears more complex. Both of these frequencies increase when the frequency of the upper kHz QPO increases from 400 to 900 Hz, but at this frequency a jump in the values of the parameters occurs. We interpret this jump in terms of the gradual appearance of a QPO at the position of the break at high inferred mass accretion rate, while the previous LFQPO disappears. Simultaneously, another kind of noise appears with a break frequency of ~7 Hz, similar to the NBO of Z sources. The 100 Hz peaked noise does not seem to correlate with the position of the source in the color-color diagram but remains relatively constant in frequency. This component may be similar to several 100 Hz QPOs observed in black hole binaries.

We investigate the properties of r-mode oscillations of a slowly rotating neutron star with a solid crust by taking account of the effects of the Coriolis force. For the modal analysis we employ three-component neutron star models that are composed of a fluid core, a solid crust, and a surface fluid ocean. For the three-component models, we find that there exist two kinds of r-modes, that is, those confined in the surface fluid ocean and those confined in the fluid core, which are most important for the r-mode instability. The r-modes do not have any appreciable amplitudes in the solid crust if rotation rate of the star is sufficiently small. We find that the core r-modes are strongly affected by mode coupling with the crustal torsional (toroidal) modes and lose their simple properties of the eigenfunction and eigenfrequency as functions of the angular rotation velocity Ω. This indicates that the extrapolation formula, which is obtained in the limit of Ω → 0, cannot be used to examine the r-mode instability of rapidly rotating neutron stars with a solid crust unless the effects of mode coupling with the crustal torsional modes are correctly taken into account.

Neutron stars with very strong surface magnetic fields have been suggested as the site for the origin of observed soft gamma repeaters (SGRs). In this paper we investigate the influence of such strong magnetic fields on the properties and internal structure of these strongly magnetized neutron stars (magnetars). We study properties of a degenerate equilibrium ideal neutron-proton-electron (npe) gas with and without the effects of the anomalous nucleon magnetic moment in a strong magnetic field. The presence of a sufficiently strong magnetic field changes the ratio of protons to neutrons as well as the neutron drip density. We also study the appearance of muons as well as pion condensation in strong magnetic fields. We discuss the possibility that boson condensation in the interior of magnetars might be a source of SGRs.

The magnetism of nuclides in the outer crusts of neutron stars is considered. Employing the shell correction treatment, we demonstrate that strong magnetic fields significantly affect shell oscillations in nuclear masses. The phase shift in shell oscillations is shown to be the main feature of the field effect. Such a phase change originates from the Pauli magnetic response associated with the relative shift of spin-up and spin-down energy levels. The neutron shell correction energy behaves almost periodically as a function of the field strength with slightly enhanced amplitude of shell oscillations at large fields. The period of the sign change is determined by the energy difference between neighbor levels. The proton shell correction energy displays an anomalous dependence on the field strength. The proton orbital magnetism enhances the nuclear shell effect especially when the field influence is comparable to the spin-orbit coupling. The nuclear magic numbers are demonstrated to depend considerably on the magnetic field on the strength scale B ~ 10¹⁶-10¹⁷ G relevant for, e.g., soft gamma repeaters and anomalous X-ray pulsars.

We propose a model for the hard X-ray (>10 keV) emission observed from the supernova remnant Cas A. Lower hybrid waves are generated in strong (mG) magnetic fields, generally believed to reside in this remnant, by shocks reflected from density inhomogeneities. These then accelerate electrons to energies of several tens of keV. Around 4% of the X-ray-emitting plasma electrons need to be in this accelerated distribution, which extends up to electron velocities of order the electron Alfvén speed and is directed along magnetic field lines. Bremsstrahlung from these electrons produces the observed hard X-ray emission. Such waves and accelerated electrons have been observed in situ at comet Halley, and we discuss the viability of the extrapolation from this case to the parameters relevant to Cas A.

The young pulsar PSR B0540-69 was one of the first targets observed with the Chandra X-Ray Observatory. The high angular resolution of Chandra allows us to resolve the compact nebula surrounding the pulsar. We have determined a position for PSR B0540-69 of R.A. = 05^h40^m11221,decl. = -69°19'5498 (J2000) with a 1 σ radial uncertainty of 07. Combining our measurements of the pulsar period with previous measurements covering a span of 12 years, we derive a braking index of 2.082 ± 0.005 (95% confidence). The spectrum of the pulsed emission is consistent with a power law with a photon index of 1.83 ± 0.13. The compact nebula has a softer spectrum with a photon index of 1.85-2.26.

On the basis of recent state-of-the-art ab initio calculations of the interatomic potential and dipole surface of interacting helium (He) and hydrogen (H) atoms, we calculate the collision-induced absorption spectra in the infrared of the He-H pair, using a rigorous quantum mechanical formalism. Furthermore, we present a simple analytical model which is capable of reproducing these calculated spectra with precision, for frequencies from 50 to roughly 10,000 cm^-1 and temperatures from 1500 to 10,000 K. For a given temperature and frequency, the ratio of the absorption coefficient and the product of the H and He densities may be evaluated in seconds, even on small computers (e.g., PCs), provided this ratio exceeds a certain (very small) lower numerical limit.

In 1977 a search for extraterrestrial intelligence at the Ohio State University Radio Observatory recorded a strong, narrowband, and apparently intermittent emission near the 21 cm hydrogen line. The detection displayed the antenna pattern signature of a transiting celestial radio source but was not repeated in subsequent transit observations. The event has been advanced by some as a candidate interstellar signal and dismissed by others as probable interference; no independent attempt to replicate the event with a spectral resolution comparable to Ohio State's has been reported. We used the Very Large Array to search for a possible underlying source—artificial or natural—which could account for the detection by occasionally brightening because of scintillation, intrinsic variability, or some other mechanism. With a sensitivity greater than 100 times the original observations, we found two continuum sources within the Ohio State coordinate error boxes, but they displayed no unusual spectral features, showed no sign of flux variability, yielded normal spectral indices based on additional observations at 6 cm, and were too weak to account for the Ohio State detection. No narrow-bandwidth point sources were detected over a band of 1.5 MHz to a flux limit of about 20 mJy at the nominal coordinates. We conclude that the "Wow" was not due to a continuous source usually below Ohio State's several jansky detection threshold but occasionally increasing in flux by a factor of less than ~100. Our search does not significantly constrain the possibility of intermittent sources because we dwelled for only 5-22 minutes per field.

The Mihalas-Däppen-Hummer (MHD) equation of state does not include the effect of relativistic partially degenerate electrons, although nonrelativistic partial degeneracy is taken into account. The discovery of a relativistic correction in helioseismology forces us to perform an appropriate upgrade of the MHD equation of state. We have adopted the method of J. M. Aparicio to evaluate the relativistic Fermi-Dirac functions. Our calculations confirm the validity of the approximation used, which works well for the weakly relativistic electrons under solar-center conditions. However, our results will also provide reliable thermodynamic quantities in the stronger relativistic regime as found in more massive stars. Since a particular feature of the original MHD papers was an explicit list of the adopted free energy and its first- and second-order analytical derivatives, we give the corresponding relativistic quantities in the Appendix.

The organic volatile composition of the long-period comet C/1999 H1 (Lee) was investigated using the first of a new generation of cross-dispersed cryogenic infrared spectrometers (NIRSPEC, at the Keck Observatory atop Mauna Kea, HI). On 1999 August 19-21 the organics spectral region (2.9-3.7 μm) was completely sampled at both moderate and high dispersion, along with the CO fundamental region (near 4.67 μm), revealing emission from water, carbon monoxide, methanol, methane, ethane, acetylene, and hydrogen cyanide. Many new multiplets from OH in the 1-0 band were seen in prompt emission, and numerous new spectral lines were detected. Several spectral extracts are shown, and global production rates are presented for seven parent volatiles. Carbon monoxide is strongly depleted in comet Lee relative to comets Hyakutake and Hale-Bopp, demonstrating that chemical diversity occurred in the giant-planets' nebular region.

We present three-dimensional MHD simulations of buoyant magnetic flux tubes that rise through a stratified model convection zone in the presence of solar rotation. The equations of MHD are solved in the anelastic approximation, and the results are used to determine the effects of solar rotation on the dynamic evolution of an Ω-loop. We find that the Coriolis force significantly suppresses the degree of fragmentation at the apex of the loop during its ascent toward the photosphere. If the initial axial field strength of the tube is reduced, then, in the absence of forces due to convective motions, the degree of apex fragmentation is also reduced. Our simulations confirm the results of thin flux-tube calculations that show the leading polarity of an emerging active region positioned closer to the equator than the trailing polarity and the trailing leg of the loop oriented more vertically than the leading leg. We show that the Coriolis force slows the rise of the tube and induces a retrograde flow in both the magnetized and unmagnetized plasma of an emerging active region. Observationally, we predict that this flow will appear to originate at the leading polarity and will terminate at the trailing polarity.

The canonical cosmological constant-dominated cold dark matter model (ΛCDM) may possess too much power on small scales at z = 0, manifested as central overconcentration of dark matter and overabundance of dwarf galaxies. We suggest an alternative model, ΛDCDM, where one-half of the cold dark matter particles decay into relativistic particles by z = 0. The model successfully lowers the concentration of dark matter in dwarf galaxies as well as in large galaxies like our own at low redshift while simultaneously retaining the virtues of the ΛCDM model. The model solves the problem of overproduction of small dwarf galaxies in the ΛCDM, not by removing them but by identifying them with failed, "dark" galaxies, where star formation is quenched as a result of dark matter evaporation and consequent halo expansion. A dramatic difference between the ΛDCDM model and other proposed variants of the ΛCDM model is that the small-scale power at high redshift (z > 2) in the ΛDCDM model is enhanced compared to the ΛCDM model. A COBE- and cluster-normalized ΛDCDM model can be constructed with the following parameters: H₀ = 60 km s^-1 Mpc^-1, λ₀ = 0.60, Ω_{0, CDM} = 0.234, Ω_{0, b} = 0.044, n = 1.0, and σ₈ = 1.06. A clean test of this model can be made by measuring the evolution of the gas fraction in clusters. The prediction is that the gas fraction should decrease with redshift and is smaller by 31% at z = 1 than at z = 0. X-ray and Sunyaev-Zeldovich effect observations should provide such a test.

Hydrodynamic simulations of Lyα clouds based on ab initio cosmological models have produced results that are in broad agreement with observations. However, further analyses have revealed that, with progressively higher numerical resolution, the median or cutoff line width of the simulated Lyα clouds (i.e., the Doppler parameter) appears to converge to a value significantly below what is observed at z ~ 3 (by about a factor of 1.5). These simulations do not include feedback from star formation. We suggest that, given the observed metallicity in the Lyα clouds, it is possible that supernovae, which presumably polluted the intergalactic medium (IGM) with metals, may have deposited a sufficient amount of energy in the IGM to reconcile the theory with observations. Simple arguments immediately narrow the redshift range of pollution down to 4 < z_dep < 9. It seems quite certain that dwarf and subdwarf galaxies with masses in the range of 10^6.5-10^9.0M_☉ have to be largely responsible for the pollution. Furthermore, it is implied that either star formation is very efficient or the metal yield is very high for these early dwarf galaxies, if the mean metallicity in the universe at z = 3 is as high as = 10^-2Z_☉.

We detected an excess of hard X-ray emission at energies above ~4 keV from the group of galaxies HCG 62 using data from the ASCA satellite. The excess emission is spatially extended up to ~10' from the group center and somewhat enhanced toward the north. Its spectrum can be represented by either a power law of photon index 0.8-2.7 or a bremsstrahlung of temperature greater than 6.3 keV. In the 2-10 keV range, the observed hard X-ray flux, (1.0 ± 0.3) × 10^-12 ergs cm^-2 s^-1, implies a luminosity of (8.0 ± 2.0) × 10⁴¹ ergs s^-1 for a Hubble constant of 50 km s^-1 Mpc^-1. The emission is thus too luminous to be attributed to X-ray binaries in the member galaxies. We discuss possible origins of the hard X-ray emission.

We report the X-ray properties of Bootes void galaxies detected by the ROSAT All-Sky Survey (RASS). By searching the fields of 26 radio and 27 emission-line-selected void galaxies, we have detected nine X-ray-emitting void galaxies at greater than 2.7 σ confidence level. Five of these nine galaxies are in the IRAS subsample. Two of the IRAS galaxies were previously identified in positional cross-correlation studies of the RASS and IRAS Point Source Catalogue sources. Three of the X-ray-emitting galaxies are active galactic nuclei (AGNs; IRAS 14288+5255, Mrk 845, and IRAS 15195+5050), three are emission-line galaxies (PC 1357+4641, CG 547, and CG 922), and the remaining three are of unknown spectral type (IRAS 14500+4804, CG 637, and IRAS 15092+3940). The far-infrared flux levels of the AGN sources imply that most of the observed X-ray emissivity is from starburst activity. We have carried out timing and spectral analysis for the Seyfert 1 galaxy Mrk 845. Poor statistics prevents detailed analysis of the remaining sources. Only two galaxies in our sample, BHI 1514+3819 and FSS 1515+3823, were observed during ROSAT pointed observations resulting in a nondetection at the 1 σ level.

The nearby Seyfert galaxy NGC 7213 has been imaged in Hα and H I with the Cerro Tololo Inter-American Observatory 1.5 m telescope and with the Australia Telescope Compact Array, respectively. Optically, NGC 7213 looks undisturbed and relatively featureless, but the continuum-subtracted Hα image shows a 19 kpc long filament located approximately 18.6 kpc from the nucleus. The Hα filament could be neutral gas photoionized by the active nucleus, as has been suggested for the Seyfert galaxy NGC 5252 or shock-ionized by a jet interacting with the surrounding H I, as has been suggested for the radio galaxy PKS 2240-41. The H I map reveals NGC 7213 to be a highly disturbed system, suggesting a past merging event.

We study the evolution and observability of young compact star clusters within ~200 pc of the Galactic center. Calculations are performed using direct N-body integration on the GRAPE-4, including the effects of both stellar and binary evolution and the external influence of the Galaxy. The results of these detailed calculations are used to calibrate a simplified model applicable over a wider range of cluster initial conditions. We find that clusters within 200 pc of the Galactic center dissolve within ~70 Myr. However, their projected densities drop below the background density in the direction of the Galactic center within ~20 Myr, effectively making these clusters undetectable after that time. Clusters farther from the Galactic center but at the same projected distance are more strongly affected by this selection effect and may go undetected for their entire lifetimes. Based on these findings, we conclude that the region within 200 pc of the Galactic center could easily harbor some 50 clusters with properties similar to those of the Arches or the Quintuplet systems.

Using data from the All-Sky Monitor (ASM) aboard the Rossi X-Ray Timing Explorer (RXTE), we found that the 1.5-12 keV X-ray count rate of Cygnus X-1 is, on timescales from 90 s to at least 10 days, strongly correlated with the spectral hardness of the source in the soft state but is weakly anticorrelated with the latter in the hard state. The correlation shows an interesting evolution during the 1996 spectral state transition. The entire episode can be roughly divided into three distinct phases: (1) a 20 day transition phase from the hard state to the soft state, during which the correlation changes from being negative to positive, (2) a 50 day soft state with a steady positive correlation, and (3) a 20 day transition back to the hard state. The pointed RXTE observations confirmed the ASM results but revealed new behaviors of the source at energies beyond the ASM passband. We discuss the implications of our findings.

We present a new observable—R_bump—which is the ratio between the star counts across the red giant branch (RGB) bump and fainter RGB stars to investigate the occurrence of a deep-mixing phenomenon during these evolutionary phases. The comparison between predicted and empirical R_bump-values, based on a large and homogeneous set of Hubble Space Telescope data, brings out that evolutionary lifetimes predicted by canonical RGB models do account for the bulk of Galactic globular clusters included in our sample (29). This evidence suggests that bump and fainter RGB stars do not show the occurrence of deep mixing, which significantly changes their chemical stratification. A few possible exceptions to this general rule are briefly discussed.

Emission from Mn II multiplet 13 (λλ6122-6132) in the spectrum of the ³He star 3 Centauri A and the hot, mild, HgMn star 46 Aquilae can be naturally explained by interlocked non-LTE effects. However, reproduction of the strength of the Mn II emission in both stars requires vertical stratification of the manganese abundance, with manganese concentrated high in the photosphere (column mass ≲10^-2 g cm^-2). If this formation picture is correct, several additional transitions of Mn II with λ > 8000 Å should also be present in emission in the spectrum of 3 Cen A. The wide range in the strength of Mn II multiplet 13 among upper main-sequence stars (ranging from absorption to emission) is made possible by the interplay in the non-LTE radiative transfer solution of the stellar T_eff, manganese abundance, and manganese stratification profile. In particular, emission is strongly suppressed by a large manganese overabundance in the photosphere. This explains why the hot, mild, HgMn star 46 Aql, which has only a modest manganese enhancement, is detected in emission in Mn II multiplet 13 while other HgMn stars of similar T_eff but with large photospheric manganese overabundances, such as κ Cancri, present Mn II multiplet 13 in absorption.

We have observed a sample of OH 1612 MHz masing objects in all four OH ground-state transitions with the Australia Telescope Compact Array. One likely post-asymptotic giant branch (AGB) object is found to emit in the 1612, 1665, and 1720 MHz transitions. We discuss the evidence that this object may be an early post-AGB object and the possibility for such a circumstellar envelope to harbor a 1720 MHz maser. We argue that during a very brief period, just after the star has left the thermally pulsing phase of the AGB and the wind velocity starts to increase, post-AGB objects might show 1720 MHz emission. The best objects to search for such emission would be those that are masing at 1612 and 1665 MHz, but not at 1667 MHz nor in the 22 GHz H₂O transition.

We report on the detection with the Infrared Space Observatory (ISO), for the first time in the circumstellar medium, of the polyacetylenic chains C₄H₂ and C₆H₂ and of benzene (C₆H₆) in the direction of the proto-planetary nebula CRL 618. Surprisingly, the abundances of di- and triacetylene are only a factor of 2-4 lower than that of C₂H₂. Benzene is ≃40 times less abundant than acetylene. We suggest that the chemistry in CRL 618 has been strongly modified by the UV photons coming from the hot central star and by the shocks associated with its high-velocity winds. All the infrared bands arise from a region with kinetic temperatures between 200 and 250 K, probably the photodissociation region associated with the dense torus that surrounds the central star. C₄H₂ and C₆H₂ have also been detected in CRL 2688, so it seems that C-rich proto-planetary nebulae are the best organic chemistry factories in space.

We report on the detection with the Infrared Space Observatory of strong infrared absorption from NH₃ and C₂H₄ in CRL 618. The observed NH₃ and C₂H₄ bands arise from a region with kinetic temperatures ≃200 K, i.e., the dense gas in the photodissociation region associated to the dense torus surrounding the central star, as was the case for the polyynes and cyanopolyynes (see the companion Letter). Several absorption bands, probably arising from small gas-phase hydrocarbons, are observed between 5.5 and 11 μm. Two of these species have been identified with the 30 m IRAM telescope as the methylpolyynes CH₃C₂H and CH₃C₄H. However, the absorption around 6.2 μm is particularly broad and could arise from the combination of these small hydrocarbons and from the aromatic C=C stretching of polycyclic aromatic hydrocarbons of moderate size. These bands and those associated to the polyynes, cyanopolyynes, methylpolyynes, and benzene are not present in the infrared spectrum of the asymptotic giant branch star IRC +10216.

Wang et al. recently described white-light coronagraph observations of faint coronal features moving inward toward the Sun at heliocentric distances of 2-6 R_☉. In a study of these inflows during 1996-2000, we have found that they occur along bends of the coronal streamer belt and are especially common when the magnetic field has a four-sector structure. The measured inflow rate is dominated by episodic bursts that are correlated with the occurrence of nonpolar coronal holes and other indicators of the Sun's nonaxisymmetric open flux. However, the inflow rate has only a broad long-term correlation with conventional indicators of solar activity like the sunspot number and coronal mass ejection rate. We conclude that most inflows indicate collapsing field lines that occur as nonpolar coronal holes are subjected to photospheric motions and the eruptions of new flux.