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YOU HAVE PROBABLY seen a picture of a compact group of galaxies without even realizing it. The angels in the opening scene of the movie It's a Wonderful Life are [...]

We employ detailed photoionization models to infer the physical conditions of intrinsic narrow absorption line systems found in high-resolution spectra of three quasars at z = 2.6-3.0. We focus on a family of intrinsic absorbers characterized by N V lines that are strong relative to the Lya lines. The inferred physical conditions are similar for the three intrinsic N V absorbers, with metallicities greater than 10 times the solar value (assuming a solar abundance pattern), and with high ionization parameters (log U [?] 0). Thus, we conclude that the unusual strength of the N V lines results from a combination of partial coverage, a high ionization state, and high metallicity. We consider whether dilution of the absorption lines by flux from the broad emission line region can lead us to overestimate the metallicities and we find that this is an unlikely possibility. The high abundances that we infer are not surprising in the context of scenarios in which metal enrichment takes place very early on in massive galaxies. We estimate that the mass outflow rate in the absorbing gas (which is likely to have a filamentary structure) is less than a few ${M_\odot \; \rm yr}^{-1}$ under the most optimistic assumptions, although it may be embedded in a much hotter, more massive outflow.

We report the results of a spectropolarimetric observation of the C VI "mini-broad" absorption line (mini-BAL) in the quasar HS 1603+3820 (z em = 2.542). The observations were carried out with the FOCAS instrument on the Subaru Telescope and yielded an extremely high polarization sensitivity of dp[?] 0.1%, at a resolving power of R [?] 1500. HS 1603+3820 has been the target of a high-resolution spectroscopic monitoring campaign for more than four years, aimed at studying its highly variable C VI mini-BAL profile. Using the monitoring observations in an earlier paper, we were able to narrow down the causes of the variability to the following two scenarios: (1) scattering material of variable optical depth redirecting photons around the absorber and (2) a variable, highly ionized screen between the continuum source and the absorber which modulates the UV continuum incident on the absorber. The observations presented here provide a crucial test of the scattering scenario and lead us to disfavor it because (1) the polarization level is very small (p [?] 0.6%) throughout the spectrum and (2) the polarization level does not increase across the mini-BAL trough. Thus, the variable screen scenario emerges as our favored explanation of the C VI mini-BAL variability. Our conclusion is bolstered by recent X-ray observations of nearby mini-BAL quasars, which show a rapidly variable soft X-ray continuum that appears to be the result of transmission through an ionized absorber of variable ionization parameter and optical depth.

Several neutral species (Mg I, Si I, Ca I, Fe I) have been detected in a weak Mg II absorption line system (Wr (2796) [?] 0.15 A) at z [?] 0.45 along the sightline toward HE0001-2340. These observations require extreme physical conditions, as noted in D'Odorico. We place further constraints on the properties of this system by running a wide grid of photoionization models, determining that the absorbing cloud that produces the neutral absorption is extremely dense ([?]100-1000 cm-3), cold (<100 K), and has significant molecular content ([?]72%-94%). Structures of this size and temperature have been detected in Milky Way CO surveys and have been predicted in hydrodynamic simulations of turbulent gas. In order to explain the observed line profiles in all neutral and singly ionized chemical transitions, the lines must suffer from unresolved saturation and/or the absorber must partially cover the broad emission line region of the background quasar. In addition to this highly unusual cloud, three other ordinary weak Mg II clouds (within densities of [?]0.005 cm-3 and temperatures of [?]10, 000 K) lie within 500 km s-1 along the same sightline. We suggest that the "bare molecular cloud," which appears to reside outside of a galaxy disk, may have had in situ star formation and may evolve into an ordinary weak Mg II absorbing cloud.

We report on the detection of a population of weak metal-line absorbers in the halo or nearby intergalactic environment of the Milky Way. Using high-resolution ultraviolet absorption-line spectra of bright quasars (QSO) obtained with the Space Telescope Imaging Spectrograph (STIS), along six sight lines we have observed unsaturated, narrow absorption in O I and Si II, together with mildly saturated C II absorption at high radial velocities (|v LSR| = 100-320 km s-1). The measured O I column densities lie in the range N(O I) < 2 x 1014 cm-2 implying that these structures represent Lyman limit Systems and sub-Lyman limit System with H I column densities between 1016 and 3 x 1018 cm-2, thus below the detection limits of current 21 cm all-sky surveys of high-velocity clouds (HVCs). The absorbers apparently are not directly associated with any of the large high column density HVC complexes, but rather represent isolated, partly neutral gas clumps embedded in a more tenuous, ionized gaseous medium situated in the halo or nearby intergalactic environment of the Galaxy. Photoionization modeling of the observed low ion ratios suggests typical hydrogen volume densities of n H > 0.02 cm-3 and characteristic thicknesses of a several parsec down to subparsec scales. For three absorbers, metallicities are constrained in the range of 0.1-1.0 solar, implying that these gaseous structures may have multiple origins inside and outside the Milky Way. Using supplementary optical absorption-line data, we find for two other absorbers Ca II/O I column-density ratios that correspond to solar Ca/O abundance ratios. This finding indicates that these clouds do not contain significant amounts of dust. This population of low column density gas clumps in the circumgalactic environment of the Milky Way is indicative of the various processes that contribute to the circulation of neutral gas in the extended halos of spiral galaxies. These processes include the accretion of gas from the intergalactic medium and satellite galaxies, galactic fountains, and outflows. We speculate that this absorber population represents the local analog of weak Mg II systems that are commonly observed in the circumgalactic environment of low- and high-redshift galaxies.

We measure the physical properties of a local multicomponent absorption-line system at V [?]200 km s-1 toward the quasar PKS 0312-770 behind the Magellanic Bridge (MB) using Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) spectroscopy in conjunction with photoionization modeling. At an impact parameter of [?]10 kpc from the Small Magellanic Cloud (SMC), this sightline provides a unique opportunity to probe the chemical properties and ionization structure in a nearby absorption line system with a column density of $\log \; N_{\rm H\; \mathsc{i}}$ [?]20.2, at the transition between damped Lya (DLA) and sub-DLA systems. We find that metallicity of -1.0 < log(Z/Z ) < -0.5 and ionization parameter of -6 < log U < -5 for three low-ionization components and log U [?] -2.6 for one high-ionization component. One component at V = 207 km s-1 shows an a-element abundance log (Si/H) [?]-5.0, making it [?]0.2 dex more metal-rich than both SMC H II regions and stars within the MB and the SMC. The N/Si ratio in this component is log (N/Si) = -0.3 +- 0.1, making it comparable to other N-poor dwarf galaxies and [?]0.2 dex lower than H II regions in the SMC. Another component at V = 236 km s-1 shows a similar Si/H ratio but has log (N/Si) = -1.0 +- 0.2, indicating a nitrogen deficiency comparable to that seen in the most N-poor DLA systems. These differences imply different chemical enrichment histories between components along the same sightline. Our results suggest that if these absorbers are representative some fraction of DLA systems, then (1) DLA systems along single sightlines do not necessarily represent the global properties of the absorbing cloud, and (2) the chemical composition within a given DLA cloud may be inhomogeneous.

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

Through photoionization modeling, constraints on the physical conditions of three z [?] 1.7 single-cloud weak Mg II systems [ Wr(2796) [?] 0.3 A] are derived. Constraints are provided by high-resolution R = 45,000, high-S/N spectra of the three quasars HE 0141-3932, HE 0429-4091, and HE 2243-6031, which we have obtained from the ESO archive of VLT UVES. Results are as follows: (1) The single-cloud weak Mg II absorption in the three z [?] 1.7 systems is produced by clouds with ionization parameters of -3.8 < log U [?] [?] 2.0 and sizes of 1-100 pc. (2) In addition to the low-ionization phase Mg II clouds, all systems need an additional one to three high-ionization phase C IV clouds within 100 km s[?]1 of the Mg II component. The ionization parameters of the C IV clouds range from -1.9 < log U < [?] 1.0, with sizes of tens of parsecs to kiloparsecs. (3) Two of the three single-cloud weak Mg II absorbers have near-solar or supersolar metallicities, if we assume a solar abundance pattern. Although such large metallicities have been found for z < 1 weak Mg II absorbers, these are the first high metallicities derived for such systems at higher redshifts. (4) Two of the three weak Mg II systems also need additional low-metallicity, broad Lya absorption lines, offset in velocity from the metal-line absorption, in order to reproduce the full Lya profile. (5) Metallicities in single-cloud weak Mg II systems are more than an order of magnitude larger than those in DLA systems at z [?] 1.7. In fact, there appears to be a gradual decrease in metallicity with increasing NH I, from these, the most metal-rich Lya forest clouds, to Lyman limit systems, to sub-DLAs, and finally to the DLAs.

We have used archival Chandra and XMM-Newton observations of quasars hosting intrinsic narrow UV absorption lines (intrinsic NALs) to carry out an exploratory survey of their X-ray properties. Our sample consists of three intrinsic NAL quasars and one "mini-BAL" quasar, plus four quasars without intrinsic absorption lines for comparison. These were drawn in a systematic manner from an optical/UV-selected sample. The X-ray properties of intrinsic NAL quasars are indistinguishable from those of "normal" quasars. We do not find any excess absorption in quasars with intrinsic NALs, with upper limits of NH[?] few x 1022 cm [?]2. We compare the X-ray and UV properties of our sample quasars by plotting the equivalent width and blueshift velocity of the intrinsic NALs and the X-ray spectral index against the "optical-to-X-ray" slope, aox. When BAL quasars and other AGNs with intrinsic NALs are included, the plots suggest that intrinsic NAL quasars form an extension of the BAL sequences and tend to bridge the gap between BAL and normal quasars. Observations of larger samples of intrinsic NAL quasars are needed to verify these conclusions. We also test two competing scenarios for the location of the NAL gas in an accretion disk wind. Our results strongly support a location of the NAL gas at high latitudes above the disk, closer to the disk axis than the dense BAL wind. We detect excess X-ray absorption only in Q0014+8118, which does not host intrinsic NALs. The absorbing medium very likely corresponds to an intervening system at z = 1.1, which also produces strong absorption lines in the rest-frame UV spectrum of this quasar. In an appendix we discuss the connection between UV and X-ray attenuation and its effect on aox.

We consider the evolution of strong [Wr(2796) > 0.3 A] Mg II absorbers, most of which are closely related to luminous galaxies. Using 20 high-resolution quasar spectra from the VLT UVES public archive, we examine 33 strong Mg II absorbers in the redshift range 0.3 < z < 2.5. We compare and supplement this sample with 23 strong Mg II absorbers at 0.4 < z < 1.4 observed previously with HIRES/Keck. We find that neither equivalent width nor kinematic spread (the optical-depth-weighted second moment of velocity) of Mg II l2796 evolve. However, the kinematic spread is sensitive to the highest velocity component, and therefore not as sensitive to additional weak components at intermediate velocities relative to the profile center. The fraction of absorbing pixels within the full velocity range of the system does show a trend of decreasing with decreasing redshift. Most high-redshift systems (14/20) exhibit absorption over the entire system velocity range, which differs from the result for low-redshift systems (18/36) at the 95% level. This leads to a smaller number of separate subsystems for high-redshift systems, because weak absorbing components tend to connect the stronger regions of absorption. We hypothesize that low-redshift Mg II profiles are more likely to represent well-formed galaxies, many of which have kinematics consistent with a disk/halo structure. High-redshift Mg II profiles are more likely to show evidence of complex protogalactic structures, with multiple accretion or outflow events. Although these results are derived from measurements of gas kinematics, they are consistent with hierarchical galaxy formation evidenced by deep galaxy surveys.

We present the results of photoionization modeling of nine weak Mg II (Wr < 0.3 A) quasar absorption-line systems with redshifts 1.4 < z < 2.4 obtained with the Ultraviolet and Visual Echelle Spectrograph on the Very Large Telescope. These systems have been chosen because they provide access to a regime of redshift space that previous weak Mg II studies have not looked at. The densities, metallicities, Doppler parameters, and column densities of these systems are compared to those of other weak Mg II systems at lower redshift. There is no significant statistical variation in the properties of the absorbers over the redshift range 0.4 < z < 2.4. The number density per unit redshift is known to decrease for weak Mg II absorbers between z ~ 1 and 2 by a greater amount than predicted from cosmological effects and changes in the extragalactic ionizing background alone. We suggest that, because the physical properties of the absorber population are not seen to change significantly across this range, the evolution in dN/dz is due to a decrease in the activity that gives rise to weak Mg II absorption, and not due to a change in the processes that form weak Mg II absorbers. The presence of separate, but aligned (in velocity) low- and high-density clouds in all single-cloud weak Mg II absorbers provides an important diagnostic of their geometry. We discuss possible origins in dwarf galaxies and in extragalactic analogs to high-velocity clouds.

We present results from a survey of weak Mg II absorbers in the VLT/UVES spectra of 81 QSOs obtained from the ESO archive. In this survey, we identified 112 weak Mg II systems within the redshift interval 0.4 < z < 2.4 with 86% completeness down to a rest-frame equivalent width of Wr(2796) = 0.02 A, covering a cumulative redshift path length of DZ ~ 77.3. From this sample, we estimate that the number of weak absorbers per unit redshift (dN/dz) increases from 1.06 +- 0.04 at [?]z[?] = 1.9 to 1.76 +- 0.08 at [?]z[?] = 1.2 and thereafter decreases to 1.51 +- 0.09 at img1.gif = 0.9 and 1.06 +- 0.10 at [?]z[?] = 0.6. Thus, we find evidence for an evolution in the population of weak Mg II absorbers, with their number density peaking at z = 1.2. We also determine the equivalent width distribution of weak systems at [?]z[?] = 0.9 and [?]z[?] = 1.9. At 0.4 < z < 1.4, there is evidence for a turnover from a power law of the form n(Wr) [?] Wimg2.gif at Wr(2796) < 0.1 A. This turnover is more extreme at 1.4 < z < 2.4, where the equivalent width distribution is close to an extrapolation of the exponential distribution function found for strong Mg II absorbers. Based on these results, we discuss the possibility that some fraction of weak Mg II absorbers, particularly single cloud systems, are related to satellite clouds surrounding strong Mg II systems. These structures could also be analogs to Milky Way high-velocity clouds. In this context, the paucity of high-redshift weak Mg II absorbers is caused by a lack of isolated clouds accreting onto galaxies during that epoch.

We present six new and two previously published high-resolution spectra of the quasar HS 1603+3820 (zem = 2.542) taken over an interval of 4.2 yr (1.2 yr in the quasar rest frame). The observations were made with the High Dispersion Spectrograph on the Subaru telescope and the Medium Resolution Spectrograph on the Hobby-Eberly Telescope. The purpose was to study the narrow absorption lines (NALs). We use time variability and coverage fraction analysis to separate intrinsic absorption lines, which are physically related to the quasar, from intervening absorption lines. By fitting models to the line profiles, we derive the parameters of the respective absorbers as a function of time. Only the mini-BAL system at zabs ~ 2.43 (vshift ~ 9500 km s-1) shows both partial coverage and time variability, although two NAL systems possibly show evidence of partial coverage. We find that all the troughs of the mini-BAL system vary in concert and its total equivalent width variations resemble those of the coverage fraction. However, no other correlations are seen between the variations of different model parameters. Thus, the observed variations cannot be reproduced by a simple change of ionization state or by motion of a homogeneous parcel of gas across the cylinder of sight. We propose that the observed variations are a result of rapid continuum fluctuations, coupled with coverage fraction fluctuations caused by a clumpy screen of variable optical depth located between the continuum source and the mini-BAL gas. An alternative explanation is that the observed partial coverage signature is the result of scattering of continuum photons around the absorber, thus the equivalent width of the mini-BAL can vary as the intensity of the scattered continuum changes.

We consider geometries and physical models for weak low-ionization absorbers based on the relative incidence of low- and high-ionization systems. We present a survey of weak low-ionization systems in 35 high-resolution HST STIS quasar spectra, sometimes supplemented by Keck HIRES and HST FOS data. We found 16 metal-line systems, with low- and/or high-ionization absorption detected. Weak low-ionization absorbers trace an abundant population of metal-enriched regions. Models show that these systems have a ~10 pc region of higher density gas and a ~1 kpc region that represents a lower density, higher ionization phase. The goal of our survey was to compare systems detected in low- and/or high-ionization gas. All but 1 of the 10 weak low-ionization systems have a related high-ionization phase. In three cases the high-ionization gas has only a single component, kinematically centered on the low-ionization absorption, and the other six cases have additional high-ionization components offset in velocity. The high-ionization absorption in weak low-ionization systems has similar kinematic structure to that in high-ionization-only systems. There are just six systems with only a high-ionization phase, as compared to the nine systems with both low- and high-ionization phases. We conclude that filamentary and sheetlike geometries are favored, due to the relatively small observed cross section of high-ionization-only systems. Our statistical arguments suggest that although low-ionization absorbers are not closely associated with luminous galaxies, they arise in their immediate environments within the cosmic web.

The exact nature of weak Mg II absorbers [those with Wr(2796) < 0.3 A] is a matter of debate, but most are likely related to areas of local star formation or supernova activity outside of giant galaxies. Using 18 QSO spectra obtained with the Ultraviolet and Visual Echelle Spectrograph (UVES) on the Very Large Telescope (VLT), we have conducted a survey for weak Mg II absorbers at 1.4 < z < 2.4. We searched a redshift path length of Dz = 8.51, eliminating regions badly contaminated by atmospheric absorption so that the survey is close to 100% complete to Wr(2796) = 0.02 A. We found a total of nine weak absorbers, yielding a number density of absorbers of dN/dz = 1.06 +- 0.12 for 0.02 [?] Wr(2796) < 0.3 A. Narayanan et al. found dN/dz = 1.00 +- 0.20 at 0 < z < 0.3, and Churchill et al. found dN/dz = 1.74 +- 0.10 at 0.4 < z < 1.4. Therefore, the population of weak Mg II absorbers appears to peak at z ~ 1. We explore the expected evolution of the absorber population subject to a changing extragalactic background radiation (EBR) from z = 0.9 to z = 1.78 (the median redshift of our survey), and find that the result is higher than the observed value. We point out that the peak epoch for weak Mg II absorption at z ~ 1 may coincide with the peak epoch of global star formation in the dwarf galaxy environment.

We present the results of a survey of the analogs of weak Mg II absorbers [rest-frame equivalent width Wr(2796) < 0.3 A] at 0 < z < 0.3. Our sample consisted of 25 HST STIS echelle quasar spectra (R = 45,000), which covered Si II l1260 and C II l1335 over this redshift range. Using those similar transitions as tracers of Mg II facilitates a much larger survey, covering a redshift path length of g(z) = 5.3 for an equivalent width limit of Mg II corresponding to Wr(2796) > 0.02 A, with 30% completeness for the weakest lines. Correcting for incompleteness, we find the number of weak Mg II absorber analogs with 0.02 A < Wr(2796) < 0.3 A to be dN/dz = 1.00 +- 0.20 for 0 < z < 0.3. This compares to a value of dN/dz = 1.74 +- 0.10 found by Churchill et al. for the higher redshift range, 0.4 < z < 1.4, and is consistent with cosmological evolution of the population. We consider the expected effect on observability of weak Mg II absorbers of the decreasing intensity of the extragalactic background radiation field from z ~ 1 to ~0. Assuming that all the objects that produce absorption at z ~ 1 are stable on a cosmological timescale and that no new objects are created, we would expect dN/dz ~ 2-3 at z ~ 0. About 30%-50% of this z ~ 0 population would be descendants of the parsec-scale structures that produce single-cloud, weak Mg II absorbers at z ~ 1. The other 50%-70% would be lower density, kiloparsec-scale structures that produce C IV absorption, but not detectable low-ionization absorption, at z ~ 1. We conclude that at least one, and perhaps some fraction of both, of these populations has evolved away since z ~ 1, in order to match the z ~ 0 value of dN/dz measured in our survey. This would follow naturally for a population of transient structures whose generation is related to star-forming processes, whose rate has decreased since z ~ 1.

We present a new spectrum of the quasar HS 1603+3820 taken 1.28 yr (0.36 yr in the quasar rest frame) after a previous observation with Subaru+HDS. The new spectrum enables us to search for time variability as an identifier of intrinsic narrow absorption lines (NALs). This quasar shows a rich complex of C IV NALs within 60,000 km s-1 of the emission redshift. On the basis of covering factor analysis, Misawa et al. found that the C IV NAL system at zabs = 2.42-2.45 (system A, at a shift velocity of vsh = 8300-10,600 km s-1 relative to the quasar) was intrinsic to the quasar. With our new spectrum, we perform time variability analysis, as well as covering factor analysis, to separate intrinsic NALs from intervening NALs for eight C IV systems. Only system A, which was identified as an intrinsic system in the earlier paper by Misawa et al., shows a strong variation in line strength (Wobs [?] 10.4 - 19.1 A). We speculate that a broad absorption line (BAL) could be forming in this quasar (i.e., many narrower lines will blend together to make a BAL profile). We illustrate the plausibility of this suggestion with the help of a simulation in which we vary the column densities and covering factors of the NAL complex. Under the assumption that a change of ionization state causes the variability, a lower limit can be placed on the electron density (ne [?] 3 x 104 cm-3) and an upper limit on the distance from the continuum source (r [?] 6 kpc). On the other hand, if the motion of clumpy gas causes the variability (a more likely scenario), the crossing velocity and the distance from the continuum source are estimated to be vcross > 8000 km s-1 and r < 3 pc. In this case, the absorber does not intercept any flux from the broad emission line region, but only flux from the UV continuum source. If we adopt the dynamical model of Murray et al., we can obtain a much more strict constraint on the distance of the gas parcel from the continuum source, r < 0.2 pc.

We present our investigation into the physical conditions of the gas in five intervening quasar absorption-line systems along the line of sight toward the quasar PG 0117+213, with redshifts of z = 0.57, 0.72, 1.04, 1.32, and 1.34. Photoionization modeling of Hubble Space Telescope, Keck I, and Palomar data, using the code Cloudy, is employed to derive densities and metallicities of the multiple phases of gas required to fit the absorption profile for each system. We discuss the implications of these models for galaxy evolution, including the interpretation of "C IV deficiency" and damped Lya absorbers (DLAs), and the relationships between galaxy morphology, galaxy luminosity, and absorption signature.

We present the results of photoionization modeling of six Mg II-selected absorption systems, at redshift 0.5 [?] z [?] 0.9, along lines of sight toward three quasars: PG 1241+176, PG 1248+401, and PG 1317+274. These are part of a larger sample of ~20 Mg II absorbers at intermediate redshift, which facilitates a survey of the properties of, and processes active in, the multiple phases of gas, both in and outside of galaxies. We present new high-resolution ultraviolet spectra from the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST), as well as high-resolution optical spectra obtained with Keck HIRES. Together, these spectra allow simultaneous study of a variety of ionization states of different chemical elements, with resolution of the kinematics providing strong constraints on multiphase properties. The six systems presented here include several that are likely to arise from lines of sight through disk and halo structures of spiral galaxies. For these we find that the high-ionization gas, traced by C IV, is consistent with coronal structures that resemble those of the Milky Way, along with high-velocity clouds at 100-200 km s-1. Another system has relatively weak C IV, relative to the low-ionization absorption, which leads to a discussion of the circumstances in which a corona might be weak or absent in a galaxy, i.e., an early-type morphology or low star formation rate. Finally, we present systems that are examples of single-cloud and multiple-cloud weak Mg II absorbers, which are as a class not likely to be within ~50 kpc of luminous galaxies. Based on the physical conditions inferred from models, we discuss the nature of these types of systems, which may trace outer regions of galaxies, dwarf galaxy satellites, or metal-rich regions of the intergalactic medium.

We present the results of a search for variability in the equivalent widths (EWs) of narrow, associated (|Dv| [?] 5000 km s-1) absorption lines found in the UV spectra of z [?] 1.5 quasars. The goal of this search was to use variability as a means of identifying absorption lines arising in gas that is intrinsic to the quasar central engine. We have compared archival Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) spectra of quasars with recent spectra obtained as part of our own snapshot survey of the same objects with the Space Telescope Imaging Spectrograph (STIS). The intervals between observations are 4-10 yr. We primarily focused on the C IV absorption lines, although we also studied other lines available in the same spectra (e.g., Lya, N V, O VI). Our main result is that 4 out of 15 quasars, or 4 out of 19 associated absorption systems, contained variable narrow absorption lines, which are indicative of intrinsic absorption. We conclude that a minimum of 21% of the associated absorption-line systems are variable. Because not all systems will have necessarily varied, this is a lower limit on this fraction and is consistent with previous estimates based on variability, partial coverage analysis, or statistical arguments. If we interpret the upper limits on the variability timescale as upper limits on the recombination time of the absorber, we constrain the density of the absorber to be ne > 3000 cm-3 and its distance from the ionizing source to be R [?] 100 pc. Moreover, we are now able to pick out specific intrinsic absorption-line systems to be followed up with high-dispersion spectroscopy in order to constrain the geometry, location, and physical conditions of the absorber. We briefly illustrate how follow-up studies can yield such constraints by means of a simulation.

We analyze high-resolution spectra of a multicloud weak [defined as Wr img1.gif < 0.3 A] absorbing system along the line of sight to PG 1634+706. This system gives rise to a partial Lyman limit break and absorption in Mg II, Si II, C II, Si III, Si IV, C IV, and O VI. The lower ionization transitions arise in two kinematic subsystems with a separation of [?]150 km s-1. Each subsystem is resolved into several narrow components having Doppler widths of 3-10 km s-1. For both subsystems, the O VI absorption arises in a separate, higher ionization phase, in regions dominated by bulk motions in the range of 30-40 km s-1. The two O VI absorption profiles are kinematically offset by [?]50 km s-1 with respect to each of the two lower ionization subsystems. In the stronger subsystem, the Si III absorption is strong, with a distinctive, smooth profile shape, and may partially arise in shock-heated gas. Moreover, the kinematic substructure of Si IV traces that of the lower ionization Mg II, but may be offset by [?]3 km s-1. On the basis of photoionization models constrained by the partial Lyman limit break, we infer a low metallicity of ~0.03 solar for the low-ionization gas in both subsystems. The broader O VI phases have a somewhat higher metallicity, and they are consistent with photoionization; the profiles are not broad enough to imply production of O VI through collisional ionization. Various models, including outer disks, dwarf galaxies, and superwinds, are discussed to account for the phase structure, metallicity, and kinematics of this absorption system. We favor an interpretation in which the two subsystems are produced by condensed clouds far out in the opposite extremes of a multilayer dwarf galaxy superwind.

We present Hubble Space Telescope/Space Telescope Imaging Spectrograph and Far Ultraviolet Spectroscopic Explorer spectra of the quasar RX J1230.8+0115 (V = 14.4,z = 0.117). In addition to Galactic, Virgo, and intervening absorption, this quasar is host to a remarkable intrinsic absorption complex. Four narrow absorption line systems, strong in C IV, N V, and O VI, lie within 5000 km s-1 of the QSO redshift. Three of the systems appear to be line locked, two in N V and two in O VI, with the common system residing in between the other two (in velocity). All three systems show signs of an intrinsic origin--smooth windlike profiles, high ionization, and partial coverage of the central engine. The fourth system, which appears at the systemic redshift of the QSO, may originate from host galaxy or intervening gas. Photoionization analyses imply column densities in the range 19.1 < log N(H) < 21 and ionization parameters in the range -1.3 < log U < 0.3. Revisiting the issue of line locking, we discuss a possible model in the context of the accretion disk/wind scenario and point out several issues that remain for future simulations and observations.

We examined the kinematics, ionization conditions, and physical size of the absorption clouds in a z = 1.3911 damped Lya absorber (DLA) in the double-image lensed quasar Q0957+561A, B (separation 135 himg1.gif pc at the absorber redshift). Using HIRES/Keck spectra (FWHM [?] 6.6 km s-1), we studied the Mg II ll2796, 2803 doublet, Fe II multiplet, and Mg I l2853 transition in absorption. Based on the Fe II profiles (the Mg II suffers from saturation), we defined six "clouds" in the system of sight line A and seven clouds in the system of sight line B. An examination of the N(v) profiles, using the apparent optical depth method, reveals no clear physical connection between the clouds in A and those in B. The observed column density ratios of all clouds are log N(Mg img2.gif)/N(Fe img2.gifimg2.gif) [?] -2 across the full ~300 km s-1 velocity range in both systems and also spatially (in both sight lines). This is a remarkable uniformity not seen in Lyman limit systems. The uniformity of the cloud properties suggests that the multiple clouds are not part of a "halo." Based on photoionization modeling, using the N(Mg img2.gif)/N(Fe img2.gifimg2.gif) ratio in each cloud, we constrain the ionization parameters in the range -6.2 [?] log U [?] -5.1, where the range brackets known abundance ratio and dust depletion patterns. The inferred cloud properties are densities of 2 [?] nH [?] 20 cm-3 and line-of-sight sizes of 1 [?] D [?] 25 pc. The masses of the clouds in system A are 10 [?] M/M [?] 1000 and in system B are 1 [?] M/M [?] 60 for spherical clouds. For planar clouds, the upper limits are 400 and 160 himg3.gif M for A and B, respectively. We favor a model of the absorber in which the DLA region itself is a single cloud in this complex, which could be a parcel of gas in a galactic interstellar medium. We cannot discern whether the H I in this DLA cloud is in a single, cold phase or in cold+warm phases. A spherical cloud of ~10 pc would be limited to one of the sight lines (A) and imply a covering factor less than 0.1 for the DLA complex. We infer that the DLA cloud properties are consistent with those of lower density, cold clouds in the Galactic interstellar medium.

A high-resolution (R = 30,000) ultraviolet spectrum is presented, which covers Lya and many low-, intermediate-, and high-ionization transitions in the three Mg II-selected absorption systems toward the quasar PG 1206+459. Three systems (A, B, and C), which are clustered within 1500 km s-1 at z ~ 0.93, were originally identified in a spectrum obtained with the High Resolution Spectrograph (HIRES) on the Keck I Telescope. A WIYN (Wisconsin-Indiana-Yale-NOAO) Gunn i-band image of the quasar field and spectroscopy of two galaxy candidates are presented. A multiphase medium is seen in all three systems, consistent with smaller, denser clouds producing low-ionization transitions (Mg II, Fe II, and Si II) and larger, diffuse, photoionized clouds giving rise to higher ionization transitions (C IV, N V, and/or O VI). (1) System A, a multicloud, weak Mg II absorber at z = 0.9254, requires a supersolar metallicity in both low- and high-ionization phases, unless an a-group enhancement is included. The low-ionization absorption is produced in clouds with sizes of 10-70 pc, which are surrounded in velocity space by broader, high-ionization components. With the unusually complex velocity structure resolved in the N V profiles, this system is unlikely to represent a traditional galaxy disk/corona. The most likely candidate host galaxy is a ~2L*, apparently warped, spiral at an impact parameter of 43 h-1 kpc. (2) System B, at z = 0.9276, has the strongest Mg II absorption and has an approximately solar metallicity in the low-ionization phase. The smooth, broad high-ionization profiles may indicate a coronal structure similar to that of the Milky Way. The redshift of an L* galaxy (z = 0.9289), at an impact parameter of 38 h-1 kpc, is consistent with the redshift of this system. (3) System C, at z = 0.9342, has a single component in Mg II, separated from the other two systems by ~+1000 km s-1. The Lya profile is not aligned with the Mg II, requiring an additional velocity component offset by -40 km s-1. System C lacks the small, low-ionization cloud characteristic of an isolated single-cloud, weak Mg II absorber. Its absorption properties are similar to the "satellite clouds" of classic strong Mg II absorbers, so this could be a high-velocity cloud in the galaxy group responsible for the systems, possibly related to a 0.2L* galaxy at an impact parameter of 43 h-1 kpc.

High-resolution optical (HIRES/Keck) and UV (STIS/Hubble Space Telescope) spectra, covering a large range of chemical transitions, are analyzed for three single-cloud weak Mg II absorption systems along the line of sight toward the quasar PG 1634+706. Weak Mg II absorption lines in quasar spectra trace metal-enriched environments that are rarely closely associated with the most luminous galaxies (>0.05L*). The two weak Mg II systems at z = 0.81 and 0.90 are constrained to have at least solar metallicity, while the metallicity of the z = 0.65 system is not as well constrained, but is consistent with more than 1/10 solar. These weak Mg II clouds are likely to be local pockets of high metallicity in a lower metallicity environment. All three systems have two phases of gas, a higher density region that produces narrower absorption lines for low-ionization transitions, such as Mg II, and a lower density region that produces broader absorption lines for high-ionization transitions, such as C IV. The C IV profile for one system (at z = 0.81) can be fitted with a single broad component (b ~ 10 km s-1), but those for the other two systems require one or two additional offset high-ionization clouds. Two possible physical pictures for the phase structure are discussed: one with a low-ionization, denser phase embedded in a lower density surrounding medium and the other with the denser clumps surrounding more highly ionized gas.

The z = 0.9902 system along the quasar PG 1634+706 line of sight is a strong Mg II absorber [Wr(l2796) > 0.3 A] with only weak C IV absorption (it is "C IV-deficient"). To study this system, we used high-resolution spectra from both the Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) and the Keck I telescope High Resolution Echelle Spectrometer (HIRES). The STIS spectrum has a resolution of R = 30,000 and covers key transitions such as Si II, C II, Si III, C III, Si IV, and C IV. The HIRES spectrum, with a resolution of R = 45,000, covers the Mg I, Mg II, and Fe II transitions. Assuming a Haardt & Madau extragalactic background spectrum, we modeled the system with a combination of photoionization and collisional ionization. Based on a comparison of synthetic spectra with the data profiles, we infer the existence of the following four phases of gas:

We have observed the broad-line radio galaxy Arp 102B with ASCA in order to determine the absorbing column density toward its X-ray source and measure its X-ray spectrum. The ultimate goal was to constrain the properties of the medium responsible for the metastable Fe II absorption lines observed in the ultraviolet spectrum of this object. The 0.5-10 keV X-ray spectrum is best described by a simple power-law model of photon index 1.58 +- 0.04 modified by photoelectric absorption with an equivalent hydrogen column density of (2.8 +- 0.3) x 1021 cm-2. An Fe Ka line is not detected with an upper limit to its equivalent width of 200 eV, assuming that its FWHM is 60,000 km s-1. Using the column density measured from the X-ray spectrum and the observed spectral energy distribution as constraints, we explore simple (single-zone, constant-density) photoionization models for the absorber for a wide range of densities and ionization parameters in an effort to reproduce the strengths of the ultraviolet absorption lines. We find that densities of at least 1011 cm-3 are needed. However, a single ionization parameter cannot explain all of the observed lines. An ionization parameter between 10-2.5 and 10-3.5 is needed to explain the Mg and Fe lines and the soft X-ray absorption, but the observed lines from Si, C, Al, and H require different density-ionization parameter combinations. According to the models, such an absorbing medium must be located very close to the source of ionizing radiation (within 5000 gravitational radii) and must be very compact. As such, the properties of this absorbing medium differ from those of more luminous quasars but are reminiscent of the absorber in the Seyfert galaxy NGC 4151. We suggest that the absorber is in the form of thin sheets or filaments embedded in an outflowing wind that overlays the accretion disk of Arp 102B. This picture is consistent with all of the available constraints on the central engine of this object. In an appendix, we present the X-ray spectrum of the source MS 1718.6+4902, which happened to fall within the field of view of the ROSAT Position Sensitive Proportional Counter and the ASCA Gas Imaging Spectrometer during the observations of Arp 102B.

We present an investigation of Mg II absorbers characterized as single-cloud "weak systems" [defined by Wr(l2796) < 0.3 A] at z ~ 1. We measured column densities and Doppler parameters for Mg II and Fe II in 15 systems found in High Resolution Echelle Spectrometer/Keck spectra at 6.6 km s-1. Using these quantities and C IV, Lya, and Lyman limit absorption observed with the Faint Object Spectrograph on the Hubble Space Telescope (resolution ~230 km s-1), we applied photoionization models to each system to constrain metallicities, densities, ionization conditions, and sizes. We find the following:

We present High Resolution Echelle Spectrometer/Keck profiles of four extremely strong (Wr > 1.8 A) Mg II absorbers at 1 < z < 2. The profiles display a common kinematic structure, having a sharp drop in optical depth near the center of the profile and strong, black-bottomed absorption on either side. This "symmetric-inverted" structure, with a velocity spread of several hundred kilometers per second, is suggestive of superwinds arising in actively star-forming galaxies. Low-ionization absorption of similar strength has been observed in local star-forming galaxies. The Mg II absorbers with Wr > 1.8 A evolve away from z = 2 to the present. We propose that a substantial fraction of these very strong absorbers are due to superwinds and that their evolution is related to the redshift evolution of star-forming galaxies. Based on the observed redshift number density of Wr > 1.8 A Mg II absorbers at 1 < z < 2, we explore whether it is realistic that superwinds from starbursting galaxies could give rise to these absorbers. Finally, we do an analysis of the superwind connection to damped Lya absorbers (DLAs). DLAs and superwinds evolve differently and usually have different kinematic structure, indicating that superwinds probably do not give rise to the majority of DLAs.

The intervening z = 0.7443 Mg II absorption system in the spectrum of MC 1331+170 shows an unusual series of line pairs, each with Dv ~ 30 km s-1. These lines could be explained as the shells of expanding superbubbles residing in the outer regions of an edge-on spiral galaxy visible in the optical image of the MC 1331+170 field. The color and brightness of this galaxy make it the most likely candidate z = 0.7443 absorber, though two other galaxies in the quasar field could also be contributing to the Mg II absorption profile. Kinematic models of absorption from compact groups and galaxy pairs produce profiles largely inconsistent with the observed Mg II spectrum. Superbubbles would naturally generate more regular structures, such as those observed. Photoionization models of the superbubble shell are consistent with the observed profile for many realistic physical conditions. In a pure superbubble model, the large velocity spread of the Mg II absorption system is inconsistent with the expected spread of a quiescent, rotating disk. This requires unusual kinematics within the host galaxy, perhaps due to a recent interaction.

Despite previous intensive ground-based imaging and spectroscopic campaigns and wideband Hubble Space Telescope (HST) imaging of the z = 0.927 QSO 3C 336 field, the galaxy that hosts the damped Lya system along this line of sight has eluded detection. We present a deep narrowband Ha image of the field of this zabs = 0.656 damped Lya absorber, obtained through the F108N filter of NICMOS1 on board the HST. The goal of this project was to detect any Ha emission 10 times closer than previous studies to unveil the damped absorber. We do not detect Ha emission between 0.''05 and 6'' (0.24 and 30 h-1 kpc) from the QSO, with a 3 s flux limit of 3.70 x 10-17 h-2 ergs s-1 cm-2 for an unresolved source, corresponding to a star formation rate (SFR) of 0.3 h-2 M yr-1. This leads to a 3 s upper limit of 0.15 M yr-1 kpc-2 on the SFR density, or a maximum SFR of 1.87 M yr-1 assuming a disk of 4 kpc in diameter. This result adds to the number of low-redshift damped Lya absorbers that are not associated with the central regions of Milky Way-like disks. Damped Lya absorption can arise from high-density concentrations in a variety of galactic environments including some that, despite their high local H I densities, are not conducive to widespread star formation.

We present an exhaustive statistical analysis of the associated (?vabs< 5000 km s-1), high-ionization (C IV, N V, O VI) narrow absorption line (NAL) systems in a sample of 59 QSOs defined from the Hubble Space Telescope(HST) QSO Absorption Line Key Project. The goals of the research were twofold: (1) to determine the frequency of associated NALs at low redshift and in low luminosity QSOs, and (2) to address the question of what QSO properties either encourage or inhibit the presence of associated NAL gas. To that end, we have compiled the QSO rest-frame luminosities at 2500 Å, 5 GHz, and 2 keV, spectral indices at 2500 Å and 5 GHz, the H? emission-line FWHM, and the radio core fraction at observed 5 GHz. In addition, we have measured the C IVemission-line FWHM. We find 17 associated NALs (16 selected by C IVand one selected by O VI) toward 15 QSOs, of which ~10 are statistically expected to be intrinsic. From a multivariate clustering analysis, we find that the QSOs group together (in parameter space) based primarily on radio luminosity, followed (in order of importance) by radio spectral index, C IVemission-line FWHM, and soft X-ray luminosity. We find that radio-loud QSOs that have compact radio morphologies, flat radio spectra [?(5 GHz) > -0.5], and mediocre C IVFWHM (6000 km s-1) do not have detectable associated NALs, down to Wr(C) = 0.35 Å. We also find that broad absorption line (BAL) QSOs have an enhanced probability of hosting detectable NAL gas. In addition, we find that the velocity distribution of associated NALs is peaked around the emission redshifts rather than the systemic redshifts of the QSOs. Finally, we find only one strong NAL [Wr(C) 1.5 Å] in our low-redshift sample. A comparison with previous higher redshift surveys reveals evolution in the number of strong NAL systems with redshift. We interpret these results in the context of an accretion disk model. We propose that NAL gas hugs the streamlines of the faster, denser, low-latitude wind, which has been associated with BALs. In the framework of this scenario, we can explain the observational clues as resulting from differences in orientation and wind properties, the latter presumably associated with the QSO radio properties.

In this paper we propose a test of the validity of a photoionization modeling technique that is applicable when a combination of high- and low-resolution spectra are available for various chemical transitions. We apply this technique to the four Mg II systems along the line of sight toward the zem = 1.335 quasar PG 1634+706 to infer the physical conditions in the multiple phases of their absorbing gas. We apply constraints from (1) High Resolution Spectrograph (HIRES/Keck I) profiles (R = 45,000 and FWHM = 6.6 km s-1) of low-ionization species Mg II and Fe II and (2) a Hubble Space Telescope (HST) archival, low-resolution Faint Object Spectrograph (FOS) spectrum (FWHM = 230 km s-1) covering Lya, Lyb, Si II, Si III, Si IV, C II, C III, C IV, and N V. For this bright quasar, very high signal-to-noise ratio (S/N) Space Telescope Imaging Spectrograph (STIS) spectra at high resolution (FWHM = 10 km s-1) were recently obtained (1999 May and June), covering 1850-3100 A. However, at the time of this writing, these data are still proprietary in the HST archive, and we deliberately present this paper without knowledge of the spectra. Since, in the near term, it is only plausible to obtain high-resolution UV spectra for a handful of the brightest quasars, it is important to determine, without any "after the fact" biases, what can be learned from low-resolution FOS data. Photoionization models of the four Mg II systems are constrained by the existing FOS and HIRES absorption profiles. In general, it is possible to constrain the low-ionization Mg II phase and to infer the presence of an additional high-ionization phase, but not to infer the detailed properties of the latter. Using some examples of consistent models, we simulate STIS high-resolution profiles of the key transitions that will be covered by the new observations, at the expected S/N. This study will serve as a fair test of the applicability of the photoionization modeling of a combination of low- and high-resolution profiles and also as an unbiased guide for extracting the detailed gaseous conditions from the forthcoming, high-resolution STIS observations. We find that the four Mg II absorbers along the PG 1634+706 line of sight exhibit a variety of Mg II kinematic structures and higher ionization phases. The z = 0.8182 system is a single-cloud, weak Mg II absorber without detected C IV in the FOS spectrum [Wr(C img1.gif) < 0.07 A at 3 s]. The new STIS observations could detect C IV to a higher sensitivity and distinguish between a broad C IV phase and C IV associated with the narrow Mg II cloud. In contrast, the z = 0.9056 absorber is a weak, single-cloud system with strong C IV absorption in the FOS spectrum; we infer that it must have supersolar metallicity, depletion of Fe relative to Mg, or an a-group-enhanced abundance pattern. In this system, three phases were required to match the FOS profiles: the narrow b ~ 3 km s-1 Mg II cloud phase, a broader b ~ 20 km s-1 phase required to produce the observed C IV, and an effective b ~ 400 km s-1 phase needed to fit the wings of the Lya line. The z = 0.9902 system is a strong Mg II absorber with five blended components; it has a relatively low metallicity, Z < -1, constrained by a strong Lyman limit break; and it also requires a broad b ~ 40 km s-1 phase to fit the observed C IV. The z = 1.0414 system has four, very weak, blended Mg II clouds and is most unusual in that two components of C IV absorption are apparent, even in the low-resolution FOS spectrum. The redward component is centered on the Mg II, but the stronger component is ~200 km s-1 to the blue of the Mg II clouds and thus requires an offset, broad, high-ionization phase.

We show that the number statistics of moderate-redshift Mg II and Lyman limit absorbers may rule out the hypothesis that high-velocity clouds are infalling intragroup material.

We examine a sample of 45 Mg II absorption-selected systems over the redshift range 0.4-1.4 in order to better understand the range of physical conditions present in the interstellar and halo gas associated with intermediate redshift galaxies. Mg II and Fe II absorption profiles were observed at a resolution of [?]6 km s-1 with HIRES/Keck. Lya and C IV data were measured in FOS spectra obtained from the Hubble Space Telescope archive (resolution [?]230 km s-1). We perform a multivariate analysis of Wr(Mg II), Wr(Fe II), Wr(C IV), and Wr(Lya) (rest-frame equivalent widths) and the Mg II kinematic spread. There is a large range of high- to low-ionization properties and kinematics in intermediate-redshift absorbers, that we find can be organized into five categories: "classic," "C IV-deficient," "single/weak," "double," and "damped Lya/H I-rich." These categories arise, in part, because there is a strong connection between low-ionization kinematics and the location of an absorber on the Wr(C IV)-Wr(Mg II) plane. Using photoionization modeling, we infer that in most absorbers a significant fraction of the C IV arises in a phase separate from that giving rise to the Mg II. We show that many of the C IV profiles are resolved in the FOS spectra because of the velocity structure in the C IV gas. For 16 systems, the galaxy MK, MB, B-K, and impact parameters are measured. We compare the available absorption-line properties (taken from Paper I) to the galaxy properties but find no significant (greater than 3 s) correlations, although several suggestive trends are apparent. We compare the locations of our intermediate redshift absorbers on the Wr(C IV)-Wr(Mg II) plane with those of lower and higher redshift data taken from the literature and find evidence for evolution that is connected with the Mg II kinematics seen in HIRES/Keck profiles of Mg II at z > 1.4. We discuss the potential of using the above categorizations of absorbers to understand the evolution in the underlying physical processes giving rise to the gas and governing its ionization phases and kinematics. We also discuss how the observed absorbing gas evolution has interesting parallels with scenarios of galaxy evolution in which mergers and the accretion of "protogalactic clumps" govern the gas physics and provide reservoirs for elevated star formation rates at high redshift. At intermediate and lower redshifts, the galaxy gaseous components and star formation rates may become interdependent and self-regulatory such that, at z [?] 1, the kinematics and balance of high- and low-ionization gas may be related to the presence of star-forming regions in the host galaxy.

From R-band images of 39 Hickson compact groups (HCGs), we use galaxy counts to determine a luminosity function extending to MR = -14.0+5 log h75, approximately 2 mag deeper than previous compact group luminosity functions. We find that a single Schechter function is a poor fit (kh^{2}_{{\nu}} $²_{ν}$ $t SUPgt 2t/SUPgt t SUBgt {ν}t/SUBgt $ $t Igt tt/Igt SUPgt 2t/SUPgt t SUBgt {ν}t/SUBgt $ >4) to the data, so we fit a composite function consisting of separate Schechter functions for the bright and faint galaxies. The bright end is best fit with M* = -21.6 and a = -0.52, and the faint end with M* = -16.1 and a = -1.17. The decreasing bright-end slope implies a deficit of intermediate-luminosity galaxies in our sample of HCGs, and the faint-end slope is slightly steeper than that reported for earlier HCG luminosity functions. Furthermore, luminosity functions of subsets of our sample reveal more substantial dwarf populations for groups with X-ray halos, groups with tidal dwarf candidates, and groups with a dominant elliptical or lenticular galaxy. Collectively, these results support the hypothesis that within compact groups, the initial dwarf galaxy population is replenished by "subsequent generations" formed in the tidal debris of giant galaxy interactions.

We model, via Monte Carlo simulations, the distribution of observed U-B, B-V, and V-I galaxy colors in the range 1.75 < z < 5 caused by variations in the line-of-sight opacity due to neutral hydrogen (H I). We also include H I internal to the source galaxies. Even without internal H I absorption, comparison of the distribution of simulated colors with the analytic approximations of Madau et al. reveals systematically different mean colors and scatter. Differences arise in part because we use more realistic distributions of column densities and Doppler parameters. However, there are also mathematical problems of applying mean and standard deviation opacities, and such application yields unphysical results. These problems are corrected using our Monte Carlo approach. Including H I absorption internal to the galaxies generally diminishes the scatter in the observed colors at a given redshift, but for redshifts of interest this diminution only occurs in the colors using the bluest bandpass. Internal column densities less than 1017 cm2 do not affect the observed colors, while column densities greater than 1018 cm2 yield a limiting distribution of high-redshift galaxy colors. As one application of our analysis, we consider the sample completeness as a function of redshift for a single spectral energy distribution (SED) given the multicolor selection boundaries for the Hubble Deep Field proposed by Madau et al. We argue that the only correct procedure for estimating the z > 3 galaxy luminosity function from color-selected samples is to measure the (observed) distribution of redshifts and intrinsic SED types and then consider the variation in color for each SED and redshift. A similar argument applies to the estimation of the luminosity function of color-selected, high-redshift QSOs.

The study of galaxy evolution using quasar absorption lines requires an understanding of what components of galaxies and their surroundings are contributing to the absorption in various transitions. This paper considers the kinematic composition of the class of 0.4 < z < 1.0 Mg II absorbers, particularly addressing the question of what fraction of this absorption is produced in halos and what fraction arises from galaxy disks. We design models with various fractional contributions from radial infall of halo material and from a rotating thick disk component. We generate synthetic spectra from lines of sight through model galaxies and compare the resulting ensembles of Mg II profiles with the 0.4 [?] z [?] 1.0 sample observed with the Keck Telescope HIRES. We apply a battery of statistical tests and find that pure disk and pure halo models can be ruled out, but that various models with rotating disk and infall/halo contributions can produce an ensemble that is nearly consistent with the data. A discrepancy in all models that we considered requires the existence of a kinematic component intermediate between halo and thick disk. The variety of Mg II profiles can be explained by the gas in disks and halos of galaxies being not very much different than galaxies in the local Universe. In any one case, there is considerable ambiguity in diagnosing the kinematic composition of an absorber from the low-ionization high-resolution spectra alone. Future data will allow galaxy morphologies, impact parameters and orientations, Fe II/Mg II of clouds, and the distribution of high-ionization gas to be incorporated into the kinematic analysis. Combining all these data will permit a more accurate diagnosis of the physical conditions along the line of sight through the absorbing galaxy.

We perform a double-line-of-sight (DLOS) analysis of the Lya forest structures that form and evolve in cosmological N-body/hydrodynamic simulations. Pairs of simulated spectra, extracted from lines of sight separated by distances from D = 12.5 kpc up to D = 800 kpc, and a "control sample" of unrelated lines of sight, are analyzed at redshifts z = 3, 2, and 1. Coincident line samples are defined for H I column density thresholds Nco = 1012.5, 1013, and 1014 cm-2. We find the following: (1) Under the assumption of a single structure size, a Bayesian analysis yields sizes that are larger for smaller Nco, and at fixed Nco the size decreases with decreasing redshift. However, these derived sizes are found to increase with increasing D, indicating that the assumption of a single structure size is invalid. (2) The column densities of coincident pairs are highly correlated for small D, with increasing scatter as D is increased, consistent with structures that have a centrally peaked NH that decreases gradually with radius. (3) The velocity difference distribution for coincident lines is very narrow for small D and widens as D is increased to meet the expectation for chance coincidences in unrelated lines of sight. This behavior is indicative of organized motion within the structures. (4) For small D, the distribution of anticoincident line column densities, Nac, falls steeply as Nac increases from the cutoff value but has a significant tail at large values, which is inconsistent with a population of spherical absorbers with sharp edges and consistent with a flattened geometry. The conclusions reached on the basis of the DLOS analysis are validated by an examination of the three-dimensional structures and velocity flows in the simulation data.