Search

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 a few times 10^22 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, alpha-ox. 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 the appendix we discuss the connection between UV and X-ray attenuation and its effect on alpha-ox., Comment: Accepted by the Astrophysical Journal

We consider the evolution of strong (W_r(2796) > 0.3A) MgII 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 MgII absorbers in the redshift range 0.3 < z < 2.5. We compare and supplement this sample with 23 strong MgII 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 MgII2796 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 absorping components tend to connect the stronger regions of absorption. We hypothesize that low redshift MgII profiles are more likely to represent well formed galaxies, many of which have kinematics consistent with a disk/halo structure. High redshift MgII 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., Comment: Accepted to the Astrophysical Journal

We present the results of photoionization modeling of nine weak MgII (W_r<0.3 Ang) quasar absorption line systems with redshifts 1.4

We present six new and two previously published high-resolution spectra of the quasar HS 1603+3820 (z_em = 2.542) taken over an interval of 4.2 years (1.2 years in the quasar rest frame). The observations were made with the High-Dispersion Spectrograph on the Subaru telescope and Medium-Resolution Spectrograph on the Hobby-Eberly Telescope. The purpose was to study the narrow absorption lines (NALs). We use time variability as well as 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 z_abs ~ 2.43 (v_off ~ 9,500 km/s) 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 nor 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., Comment: 29 pages, including 17 figures; accepted for publication in the Astrophysical Journal

We present results from a survey of weak MgII absorbers in the VLT/UVES spectra of 81 QSOs obtained from the ESO archive. In this survey, we identified 112 weak MgII systems within the redshift interval 0.4 < z < 2.4 with 86% completeness down to a rest-frame equivalent width of W_r(2796) = 0.02A, covering a cumulative redshift path length of deltaZ=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 =1.9 to 1.76 +/- 0.08 at =1.2 and thereafter decreases to 1.51 +/- 0.09 at =0.9 and 1.06 +/- 0.10 at =0.6. Thus we find evidence for an evolution in the population of weak MgII absorbers, with their number density peaking at z=1.2. We also determine the equivalent width distribution of weak systems at =0.9 and =1.9. At 0.4 < z < 1.4, there is evidence for a turnover from a powerlaw of the form n(W_r) \propto W_r^{-1.04} at W_r(2796) < 0.1A. 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 MgII absorbers. Based on these results, we discuss the possibility that some fraction of weak MgII absorbers, particularly single cloud systems, are related to satellite clouds surrounding strong MgII systems. These structures could also be analogs to Milky Way high velocity clouds. In this context, the paucity of high redshift weak MgII absorbers is caused by a lack of isolated accreting clouds on to galaxies during that epoch., Comment: 14 pages, 11 figures, ApJ accepted

We compare a new R = 120,000 spectrum of PG1634+706 (z_QSO = 1.337,m_V = 14.9) obtained with the HDS instrument on Subaru to a R = 45, 000 spectrum obtained previously with HIRES/Keck. In the strong MgII system at z = 0.9902 and the multiple cloud, weak MgII system at z = 1.0414, we find that at the higher resolution, additional components are resolved in a blended profile. We find that two single-cloud weak MgII absorbers were already resolved at R = 45,000, to have b = 2 - 4 km/s. The narrowest line that we measure in the R = 120, 000 spectrum is a component of the Galactic NaI absorption, with b = 0.90+/-0.20 km/s. We discuss expectations of similarly narrow lines in various applications, including studies of DLAs, the MgI phases of strong MgII absorbers, and high velocity clouds. By applying Voigt profile fitting to synthetic lines, we compare the consistency with which line profile parameters can be accurately recovered at R = 45,000 and R = 120,000. We estimate the improvement gained from superhigh resolution in resolving narrowly separated velocity components in absorption profiles. We also explore the influence of isotope line shifts and hyperfine splitting in measurements of line profile parameters, and the spectral resolution needed to identify these effects. Super high resolution spectra of quasars, which will be routinely possible with 20-meter class telescopes, will lead to greater sensitivity for absorption line surveys, and to determination of more accurate physical conditions for cold phases of gas in various environments., Comment: To appear in AJ. Paper with better resolution images available at http://www.astro.psu.edu/users/anand/superhigh.AJ.pdf

We consider geometries and possible physical models for weak low ionization absorbers based on the relative incidence of low and high ionization absorption systems. We found a total of 16 metal-line systems, with low and/or high ionization absorption detected in our survey of weak low ionization absorption systems from the archive of HST/STIS data. The weak low ionization absorbers trace an abundant population of metal-enriched regions (close to solar metallicity). Generally, models show that these systems have a ~10pc region of higher density gas and a ~1kpc region of lower density phase of higher ionization absorption. We compare absorption systems detected in low and/or high ionization gas and find the following: 1) All but 1 of the 10 weak low ionization systems have a related high ionization phase. In 3 cases the high ionization gas has only a single component, kinematically centered on the low ionization absorption, and in the other 6 cases there are additional high ionization components offset in velocity. The system, toward quasar 3C 273, do not have a high ionization cloud; 2) There are just 6 systems with only a high ionization phase as compared to the 9 systems with both low and high ionization phases; 3) The high ionization absorption in weak low ionization systems is, on average, stronger than in systems with only high ionization absorption; 4) The kinematic structure of the high ionization in weak low ionization systems is similar to that in high ionization only systems. We find that filamentary and sheetlike geometries are favored, due to the relatively small observed cross-section of high ionization only systems. Although low ionization absorbers are not closely associated with luminous galaxies, they arise in their immediate environments within the cosmic web., Comment: 25 pages, 19 figures, 4 tables

The exact nature of weak MgII absorbers (those with W_r(2796) < 0.3 A) is a matter of debate, but most are likely related to areas of local star formation or supernovae activity outside of giant galaxies. Using 18 QSO spectra obtained with the Ultra-Violet Echelle Spectrograph (UVES) on the Very Large Telescope (VLT), we have conducted a survey for weak MgII absorbers at 1.4 < z < 2.4. We searched a redshift path length of 8.51, eliminating regions badly contaminated by atmospheric absorption so that the survey is close to 100% complete to W_r(2796) = 0.02 A. We found a total of 9 weak absorbers, yielding a number density of absorbers of dN/dz = 1.06 +/- 0.12 for 0.02 <= W_r(2796) < 0.3 A. Narayanan et al. (2005) found dN/dz = 1.00 +/- 0.20 at 0 < z < 0.3 and Churchill et al. (1999) found dN/dz = 1.74 +/- 0.10 at 0.4 < z < 1.4. Therefore, the population of weak MgII 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 MgII absorption at z~1 may coincide with the peak epoch of global star formation in the dwarf galaxy environment., Comment: 25 pages, 14 figures, to be published in ApJ 01 March 2006, v639, 1

We present the results of a survey of the analogs of weak MgII absorbers (rest frame equivalent width W(2796) < 0.3 A) at 0 < z < 0.3. Our sample consisted of 25 HST/STIS echelle quasar spectra (R = 45,000) which covered SiII 1260 and CII 1335 over this redshift range. Using those similar transitions as tracers of MgII facilitates a much larger survey, covering a redshift pathlength of g(z) = 5.3 for an equivalent width limit of MgII corresponding to W(2796) > 0.02 A, with 30% completeness for the weakest lines. We find the number of weak MgII absorber analogs with 0.02 < W(2796) < 0.3 to be dN/dz = 1.00 +/- 0.20 for 0 < z < 0.3. This value is consistent with cosmological evolution of the population. We consider the expected effect on observability of weak MgII absorbers of the decreasing intensity of the extragalactic background radiation eld from z~1 to z~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 of 2-3 at z~0. About 30-50% of this z~0 population would be decendants of the parsec-scale structures that produce single-cloud, weak MgII absorbers at z~1. The other 50-70% would be lower density, kiloparsec-scale structures that produce CIV 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 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., Comment: 45 pages, 12 figures, 7 tables ApJ accepted

We present a new spectrum of the quasar HS1603+3820 taken 1.28 years (0.36 years 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 of the emission redshift. Based on covering factor analysis, Misawa et al. found that the C IV NAL system at z_abs= 2.42--2.45 (System A, at a shift velocity of v_sh = 8,300--10,600 km/s 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 8 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 (W_obs ~ 10.4A -> 19.1A). We speculate that a broad absorption line (BAL) could be forming in this quasar. We illustrate the plausibility of this suggestion with the help of a simulation. Under the assumption that a change of ionization state causes the variability, a lower limit can be placed on the electron density (n_e > 3x10^4 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 v_cross > 8,000 km/s 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., Comment: 18 pages, including 17 figures; accepted for publication in the Astrophysical Journal

We present the following results of photoionization modeling of six MgII-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 MgII absorbers at intermediate redshift, that 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 HST/STIS as well as high resolution optical spectra obtained with HIRES/Keck. 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 CIV is consistent with coronal structures that resemble that of the Milky Way, along with high velocity clouds at 100-200 km/s. Another system has relatively weak CIV, 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 MgII absorbers, which are as a class not likely to be within ~50 kpc of luminous galaxies. Based 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., Comment: 31 pages, 6 figures, 9 tables, accepted to Astrophysical Journal

We analyze high resolution spectra of a multi--cloud weak [defined as W_r(MgII) < 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 MgII, SiII, CII, SiIII, SiIV, CIV, and OVI. The lower ionization transitions arise in two kinematic subsystems with a separation of ~150 km/s. Each subsystem is resolved into several narrow components, having Doppler widths of 3-10 kms. For both subsystems, the OVI absorption arises in a separate higher ionization phase, in regions dominated by bulk motions in the range of 30-40 km/s. The two OVI absorption profiles are kinematically offset by ~50 km/s with respect to each of the two lower ionization subsystem. In the stronger subsystem, the SiIII absorption is strong with a distinctive, smooth profile shape and may partially arise in shock heated gas. Moreover, the kinematic substructure of SiIV traces that of the lower ionization MgII, but may be offset by ~3 km/s. Based upon 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 OVI phases have a somewhat higher metallicity, and they are consistent with photoionization; the profiles are not broad enough to imply production of OVI 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 multi-layer dwarf galaxy superwind.

We present HST/STIS and FUSE spectra of the quasar RXJ1230.8+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 CIV, NV, and OVI, lie within 5000 km/s of the QSO redshift. Three of the systems appear to be line-locked, two in NV, and two in OVI, with the common system residing in between the other two (in velocity). All three systems show signs of an intrinsic origin -- smooth wind-like 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

High resolution rest frame UV quasar absorption spectra covering low and high ionization species, as well as the Lyman series lines, provide remarkably detailed information about the gaseous phases of galaxies and their environments. For redshifts less than 1.5, many important chemical transitions remain in the observed ultraviolet wavelength range. I present examples of absorption that arises from lines of sight through a variety of structures, drawn from UV spectra recently obtained with STIS/HST. Even with the greater sensitivity of COS/HST there will be a limit to how many systems can be studied in detail. However, there is a great variety in the morphology of the phases of gas that we observe, even passing through different regions of the same galaxy. In order to compile a fair sample of the gaseous structures present during every epoch of cosmic history, hundreds of systems must be sampled. Multiple lines of sight through the same structures are needed, as well as some probing nearby structures whose luminous hosts have been studied with more standard techniques. Combined with high resolution optical and near--IR ground--based spectra, it will be possible to uniformly study the gaseous morphologies of galaxies of all types through their entire evolutionary histories., Comment: 8 pages, 3 figures, to appear in "Hubble's Science Legacy: Future Optical-Ultraviolet Astronomy from Space", eds. K.R. Sembach, J.C. Blades, G.D. Illingworth, & R.C. Kennicutt, ASP Conference Series

A new and mysterious class of object has been revealed by the detection of numerous weak MgII doublets in quasar absorption line spectra. The properties of these objects will be reviewed. They are not in close proximity to luminous galaxies, yet they have metallicities close to the solar value; they are likely to be self-enriched. A significant fraction of the weak MgII absorbers are constrained to be less than 10 parsecs in size, yet they present a large cross-section for absorption, indicating that there are more than a million times more of them than there are luminous galaxies. They could be remnants of Population III star clusters or tracers of supernova remnants in a population of "failed dwarf galaxies" expected in cold dark matter structure formation scenarios., Comment: 10 pages, 6 figures, To appear in "Extragalactic Gas at Low Redshift", ASP Proceedings, TBD, ed. J. Mulchaey

We present an investigation of MgII absorbers characterized as single-cloud weak systems at z~1. We measured column densities and Doppler parameters for MgII and FeII in 15 systems found in HIRES/Keck spectra at 6.6 km/s. Using these quantities and CIV, Lyman alpha and Lyman limit absorption observed with FOS/HST (resolution ~230 km/s) we applied photoionization models to each system to constrain metallicities, densities, ionization conditions, and sizes. We find that: (1) Single-cloud weak systems are optically thin in neutral hydrogen and may have their origins in a population of objects distinct from the optically thick strong MgII absorbers, which are associated with bright galaxies. (2) Weak systems account for somewhere between 25% to 100% of the z < 1 Lyman alpha forest clouds in the range 15.8

We present an ultraviolet spectrum of MRC 2251-178 taken with HST/STIS-G230L. The observation is part of a snap shot program of QSOs to systematically search for intrinsic absorption lines through variability. The sample consists of all QSOs observed with HST/FOS which showed associated narrow absorption lines. The FOS spectrum of MRC 2251-178, taken in 1996, showed an associated C IV doublet with an C IV \lambda1548 equivalent width 1.09+/-0.09 A (Monier et al. 2001). It is not detected in the STIS spectrum taken four years later, down to a 3-sigma threshold of 0.19 A. In addition to its other accolades, these observations make MRC 2251-178 the very first QSO at low redshift in which the associated absorption is shown to be truly intrinsic. We discuss the implications of this and suggest courses for future study., Comment: 9 pages total, 2 figures, accepted for publication in The Astrophysical Journal Letters, also available at http://www.astro.psu.edu/users/ganguly/ under Related Publications

We present two low resolution (R~1300) high signal-to-noise spectra (Wr,lim~50mA) of the quasar CSO 118 (zem=2.97) taken with the Hobby-Eberly Telescope Marcario Low Resolution Spectrograph. We detect eight absorbers selected by the CIV 1548,1550 absorption doublet in the redshift range {2.23 < zabs < 2.97}, seven of which are zabs > 2.68. In the redshift range covered by the seven zabs > 2.68 systems, one expects to find two absorbers. We discuss possible explanations for such an excess of absorbers in a small velocity range. Only superclustering at high redshift and absorption due to intrinsic gas are feasibly allowed., Comment: 12 pages total, 2 tables, 2 figures, accepted for publication in ApJ Letters

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 velocity separations of 30 km/s. 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., Comment: Five Figures; Two Tables; 12 Pages using emulateapj.sty; Submitted to ApJ

Despite previous intensive ground-based imaging and spectroscopic campaigns and wide-band HST imaging of the z=0.927 QSO 3C336 field, the galaxy that hosts the damped Ly-alpha system along this line-of-sight has eluded detection. We present a deep narrow-band H-alpha image of the field of this z=0.656 damped Ly-alpha absorber, obtained through the F108N filter of NICMOS 1 onboard the Hubble Space Telescope. The goal of this project was to detect any H-alpha emission 10 times closer than previous studies to unveil the damped absorber. We do not detect H-alpha emission between 0.05'' and 6'' (0.24 and 30 $h^{-1}$ kpc) from the QSO, with a 3-sigma flux limit of $3.70 \times 10^{-17} h^{-2}$ erg/s/cm^2 for an unresolved source, corresponding to a star formation rate (SFR) of $0.3 h^{-2}$ M_sun/yr. This leads to a 3-sigma upper limit of 0.15 M_sun/yr/kpc^2 on the SFR density, or a maximum SFR of 1.87 M_sun/yr assuming a disk of 4 kpc in diameter. This result adds to the number of low redshift damped Ly-alpha absorbers that are not associated with the central regions of Milky-Way-like disks. Damped Ly-alpha absorption can arise from high density concentrations in a variety of galactic environments including some that, despite their high local HI densities, are not conducive to widespread star formation., Comment: 18 pages, 3 figures. Replaced to match published version in ApJ, 550, 585 (Apr 1 2001)

We present a statistical analysis of the associated, high ionization narrow absorption line (NAL) systems in a sample of 59 QSOs defined from the HST QSO Absorption Line Key Project. We have compiled the QSO luminosities at 2500 A, 5 GHz, and 2 keV, spectral indices at 2500 A and 5 GHz, the H-beta emission line FWHM, the CIV emission line FWHM, and the radio core fraction. We find 17 associated NALs toward 15 QSOs, of which ~10 are statistically expected to be intrinsic. From a multivariate clustering analysis, the QSOs seem to group together based primarily on radio luminosity, followed by radio spectral index, CIV emission line FWHM, and soft X-ray luminosity. Radio-loud QSOs which have compact radio morphologies, flat radio spectra, and mediocre CIV FWHM (<6000 km/s) do not have detectable associated NALs, down to REW(CIV)=0.35 A. We find that BALQSOs have an enhanced probability of hosting detectable NAL gas and that the velocities of associated NALs are peaked around the emission redshifts, rather than the systemic redshifts of the QSOs. There is only one strong NAL [REW(CIV)>1.5 A] in our low redshift sample which implies evolution in the number of strong NALs with redshift. We interpret these results in the context of an accretion-disk model. In this framework, we suggest that the observational clues result from differences in orientation and wind properties, the latter presumably associated with the QSO radio properties. (abridged), Comment: Accepted for publication in ApJ, 20 pages text, 13 figures, 6 tables, 43 pages total, uses AASTeX 5.0, also available at http://www.astro.psu.edu/users/ganguly/ under Related Publications

We briefly review, at a level appropriate for graduate students and non-specialists, the field of quasar absorption lines (QALs). Emphasis is on the intervening absorbers. We present the anatomy of a quasar spectrum due to various classes of intervening absorption systems, and a brief historical review of each absorber class (Lyman-alpha forest and Lyman limit systems, and metal-line and damped Lyman-alpha absorbers). We also provide several heuristic examples on how the physical properties of both the intergalactic medium and the gaseous environments associated with earlier epoch galaxies can be inferred from QALs. The evolution of these environments from z=4 are discussed., Comment: 15 pages, 9 figures; Written for the Encyclopedia of Astronomy and Astrophysics (to be published in 2000 by Mac Millan and the Institute of Physics Publishing)

[Abridged] We examine a sample of 45 Mg II absorption-selected systems over the redshift range 0.4 to 1.4. Mg II and Fe II absorption profiles were observed at a resolution of ~6 km/s with HIRES/Keck. Lyman-alpha and C IV data were measured in archival FOS/HST spectra (~230 km/s). We perform a multivariate analysis of W(MgII), W(FeII), W(CIV) and W(Lya) (rest-frame) equivalent widths and the Mg II kinematic spread. We find five categories of Mg II absorbers: "Classic", "C IV-deficient", "Single/Weak", "Double", and "DLA/HI-Rich". There is a strong connection between low-ionization kinematics and the location of an absorber on the W(CIV)-W(MgII) plane. In most absorbers a significant fraction of the C IV arises in a phase separate from Mg II. Many of the C IV profiles are resolved in the FOS spectra due to velocity structure.. For 16 galaxies, we compare the available absorption-line properties (taken from Churchill et al. 2000, Paper I) to the galaxy properties but find no significant (greater than 3-sigma) correlations, although several suggestive trends are apparent. We compare the locations of our intermediate redshift absorbers on the W(CIV)-W(MgII) 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. 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., Comment: Accepted: The Astrophysical Journal; Work based upon data presented in Paper I [astro-ph/0005585]

We present extensive metal-line absorption properties for 45 absorption systems that were selected by their Mg II absorption at redshifts between 0.4 and 1.4. For each system the properties of several chemical species are determined, including a wide range of ionization conditions. In the optical, the absorption systems have been observed at ~6 km/s resolution with HIRES/Keck, which covered Mg II, several Fe II transitions, Mg I, and in some cases (depending upon redshift), Ca II, Ti II, Mn II, and Al III. Ultraviolet, lower resolution (~230 km/s) Faint Object Spectrograph data (1600 - 3275 Ang) were obtained from the Hubble Space Telescope archive. These spectra covered Al II, Al III, Si II, Si III, Si IV, C II, C III, C IV, N V, O VI, and several Lyman series transitions, with coverage dependent upon the absorption system redshift. From these data, we infer that Mg II absorbing galaxies at intermediate redshifts have multiphase gaseous structures., Comment: Accepted: The Astrophysical Journal

We show that the number statistics of moderate redshift MgII and Lyman limit absorbers may rule out the hypothesis that high velocity clouds are infalling intragroup material., Comment: 4 pages, no figures; submitted to Astrophysical Journal Letters; revised version, more general and includes more about Braun and Burton CHVCs

In the course of a large intervening MgII survey of 25 quasars, the CIV emission lines of six (three radio-loud and three radio-quiet) were observed serendipitously. In four of these six quasars, we detected associated narrow absorption lines. Three of these four quasars are radio-quiet while the other is radio-loud. As a result of the original target selection, the small sample of six quasars is unbiased towards finding associated CIV absorption lines. In three of these four cases, the absorption line optical-depth ratios deviate from the prediction based on atomic physics, suggesting that the background photon source(s) are only partially covered by the absorbing medium and, by extension, that the absorption lines are intrinsic to the quasar. We have used the method of Barlow & Sargent to determine the effective coverage fraction of background source(s) and we have extended it to constrain the coverage fraction of the continuum and broad-emission line sources separately. We have also applied this refined method to the narrow intrinsic absorption lines in three additional quasars for which the necessary data were available from the literature. We find that in two objects from our sample, the continuum source must be partially covered regardless of the covering factor of the emission-line source. We discuss these results in the context of the properties of absorption lines observed in different types of active galaxies and related outflow phenomena. We cannot distinguish between possible mechanisms for the origin of the partial coverage signature although we do consider possible observational tests. Finally, we speculate on how the gas responsible for the narrow lines may be related to the accretion disk wind that may be responsible for the broad absorption lines observed in some quasars., Comment: 34 pages including figures, accepted by AJ

We model, via Monte Carlo simulations, the distribution of observed U-B, B-V, V-I galaxy colors in the range 1.75 10^18 cm^2 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 multi-color selection boundaries for the Hubble Deep Field proposed by Madau et al (1996). 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., Comment: accepted for publication in ApJ; 25 pages text, 14 embedded figures

One of the external fields that influences the population of globular clusters is that due to galactic bulges. In extreme situations, perigalactic distances $r_p \le 100$ pc, globular clusters could suffer total disruption in a single passage. A more common scenario is that for cluster orbits with $r_p \ge 200$ pc. We investigate the effects of tidal forces from a bulge on the shape of globular clusters for this type of encounters. We find distortions characterized by ``twisting isophotes'' and consider the potential for observability of this effect. In the Milky Way, a typical globular cluster must pass within several hundred pc of the center to experience substantial distortion, and it is possible that this has happened recently to one or two present day clusters. We estimate that this distortion could be observed even for globulars in dense fields toward the bulge. In more extreme environments such as giant ellipticals or merger products with newly formed globulars, this effect could be more common, extending out to orbits that pass within 1 kpc of the bulge center. This would lead to a substantial shift in the eccentricity distribution of globulars in those galaxies., Comment: 12 pages, 8 figures

We used HIRES/Keck profiles (R=6 km/s) of MgII and FeII in combination with FOS/HST spectra (R=230 km/s) to place constraints on the physical conditions (metallicities, ionization conditions, and multi-phase distribution) of absorbing gas in three galaxies at z=0.9254, 0.9276, and 0.9343 along the line of sight to PG 1206+459. The chemical and ionization species covered in the FOS/HST spectra are HI, SiII, CII, NII, FeIII, SiIII, SiIV, NIII, CIII, CIV, SVI, NV, and OVI, with ionization potentials ranging from 13.6 to 138 eV. The multiple MgII clouds exhibit complex kinematics and the CIV, NV and OVI are exceptionally strong in absorption. We assumed that the MgII clouds are photoionized by the extra-galactic background and determined the allowed ranges of their physical properties as constrained by the absorption strengths in the FOS spectra. A main result of this paper is that the low resolution spectra can provide meaningful constraints on the physical conditions of the MgII clouds, including allowed ranges of cloud to cloud variations within a system. We find that the MgII clouds, which have a typical size of ~100 pc, give rise to the SiIV, the majority of which arises in a single, very large (~5 kpc), higher ionization cloud. However, the MgII clouds cannot account for the strong CIV, NV, and OVI absorption. We conclude that the MgII clouds are embedded in extended (10-20 kpc), highly ionized gas that gives rise to CIV, NV, OVI; these are multi-phase absorption systems. The high ionization phases have near-solar metallicity and are consistent with Galactic-like coronae surrounding the individual galaxies, as opposed to a very extended common "halo" encompassing all three galaxies., Comment: Accepted for publication in the Astronomical Journal

We present a search for "weak" MgII absorbers [those with W_r(2796) < 0.3 A in the HIRES/Keck spectra of 26 QSOs. We found 30, of which 23 are newly discovered. The spectra are 80% complete to W_r(2796) = 0.02 A and have a cumulative redshift path of ~17.2 for the redshift range 0.4 < z < 1.4. The number of absorbers per unit redshift, dN/dz, is seen to increase as the equivalent width threshold is decreased; we obtained dN/dz = 1.74+/-0.10 for our 0.02 <= W_r(2796) < 0.3 A sample. The equivalent width distribution follows a power law with slope -1.0; there is no turnover down to W_r(2796) = 0.02 A at = 0.9. Weak absorbers comprise at least 65% of the total MgII absorption population, which outnumbers Lyman limit systems (LLS) by a factor of 3.8+/-1.1; the majority of weak MgII absorbers must arise in sub-LLS environments. Tentatively, we predict that ~5% of the Lyman-alpha forest clouds with W_r(1215) > 0.1 A will have detectable MgII absorption to W_r,min(2796) = 0.02 A and that this is primarily a high-metallicity selection effect (Z/Z_sun] > -1). This implies that MgII absorbing structures figure prominently as tracers of sub-LLS environments where gas has been processed by stars. We compare the number density of W_r(2796) > 0.02 A absorbers with that of both high and low surface brightness galaxies and find a fiducial absorber size of 35h^-1 to 63h^-1 kpc, depending upon the assumed galaxy population and their absorption properties. The individual absorbing "clouds" have W_r(2796) <= 0.15 A and their narrow (often unresolved) line widths imply temperatures of ~25,000 K. We measured W_r(1548) from CIV in FOS/HST archival spectra and, based upon comparisons with FeII, found a range of ionization conditions (low, high, and multi-phase) in absorbers selected by weak MgII., Comment: Accepted Version: 43 pages, PostScript figures embedded; accepted to ApJ; updated version includes analysis of CIV absorption

From R-band images of 39 Hickson compact groups (HCGs), we use galaxy counts to determine a luminosity function extending to M_R=-14.0, approximately two magnitudes deeper than previous compact group luminosity functions. We find that a single Schechter function is a poor fit 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 alpha=-0.52 and the faint end with M^*=-16.1 and alpha=-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., Comment: 26 pages, to be published in The Astrophysical Journal, 8 greyscale plates (figures 1 and 2) can be retrieved at http://www.astro.psu.edu/users/sdh/pubs.html

In the z=1.94 CIV absorption line system in the spectrum of quasar Q1222+228 (z_em=2.04), we find two clouds which have contrasting physical conditions, although they are only at a 17 km/s velocity separation. In the first cloud SiII, SiIV, and CII are detected, and AlII and AlIII column density limits in conjunction with photoionization models allow us to infer that this cloud has a large Si abundance and a small Al abundance relative to a solar abundance pattern. This pattern resembles that of Galactic metal-poor halo stars, which must have formed from such high redshift gas. The second cloud, in contrast, has detected AlII and AlIII (also SiIV and CII), but no detected SiII. We demonstrate, using photoionization models, that Al/Si must be greater than (Al/Si)_Sun in this unusual cloud. Such a ratio is not found in absorption profiles looking through Milky Way gas. It cannot be explained by dust depletion since Al depletes more severely than Si. Comparing to other Al-rich environments, we speculate about the processes and conditions that could give rise to this abundance pattern., Comment: 10 pages, 2 figures, AASTEX (aaspp4), to be published in ApJL

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 MgII 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 MgII profiles with the 0.4 < z < 1.0 sample observed with HIRES/Keck. 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 MgII profiles can be explained by the gas in disks and halos of galaxies 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, FeII/MgII 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., Comment: 34 pages including 14 postscript figures; Accepted by the Astrophysical Journal; URL http://www.astro.psu.edu/users/cwc/pubs.html

Based upon QSO absorption line studies, galaxies at 0.3 < z < 1.0 are seen to have extended gaseous "halos". When the absorption lines are observed at high resolution, the velocity distribution of the absorbing gas gives clues as to where this gas actually arises in and around the galaxies. Cloud to cloud abundance ratios provide further clues, given that the ionization conditions and chemical evolution histories of outer halos clouds, spiral disk clouds, and clouds in elliptical galaxies are different. The key to interpreting the high redshift data is the Milky Way galaxy, the absorption line "Rosetta stone.", Comment: 3 pages; Latex; paspconf.sty; figures included; To appear in the Proceedings of the UC Santa Cruz Workshop on Galactic Halos (ed. D. Zaritsky)

A consensus is developing on the nature of various populations of absorbers at different redshift regimes and in particular on their relationships to galaxies at those epochs. As one example we discuss the population of z < 1 MgII absorbers. Kinematic models are presented that show this population to be consistent with some combination of halo and disk kinematics and not with a pure disk or halo model. In contrast, the 2 < z < 3 CIV absorbers are likely to represent a mix of already formed halos and protogalactic clumps in the process of assembly. Clues could be provided by the level at which SiIV traces the same kinematic components as CIV (relative to how it traces lower ionization gas) and this is illustrated by a simple application of principal components analysis. It remains uncertain how much unambiguous information it will be possible to extract about the detailed properties of the gas along the line of sight through an individual absorption profile. However, ratios of various low and high ionization transitions along the line of sight can be combined with insights gained from kinematic models and from observations of galaxy properties. There is reason for optimism that we will progress toward a point when we will someday study the interstellar medium and gaseous halos and environments of absorbing galaxies at close to the same level as we study these components in the Milky Way., Comment: 6 pages, 4 EPS figures, conf_iap.sty. To appear in 13th IAP Workshop (1-5 July 1997) "Evolution of the Intergalactic Medium From QSO Absorption Line Systems"

To understand which parts of galaxies give rise to the variety of observed MgII absorption profiles seen in QSO spectra, we have embarked upon a program to generate simulated absorption profiles as they would arise from the many typical galactic structures we see today. Here, we present preliminary results for a clumpy, rotating disk which has been sampled by a variety of QSO lines of sight. The resulting ensemble of simulated profiles is qualitatively similar to those observed in HIRES spectra of 0.4 < z < 1.0 MgII systems (Churchill, this volume). Additionally, we discuss the statistical contribution from satellite galaxies, which would be expected to contribute additional absorption subcomponents and are likely to have been more numerous at intermediate redshifts. Our preliminary finding is that the common disk and satellite components of galaxies may, to a large degree, be the structures giving rise to a variety of the observed MgII absorption profiles., Comment: 4 pages, Latex file, and 1 PostScript figure; To appear in IAGUSP Workshop on Young Galaxies and QSO Absorbers, eds. S. M. Viegas, R. Gruenwald, and R. de Carvalho, (PASP Conference Series); Available as a single PostScript file at http://www.ucolick.org/~cwc/qso/abstract.html

Quasar absorption lines provide detailed information on the chemical, kinematic, and ionization conditions in galaxies and their environments, and provide a means for studying the evolution of these conditions back to the epoch of the first quasars. Among the collection of absorbing structures along the lines of sight to quasars there is an evolutionary sequence of galaxies that represent predecessors of the Milky Way and provide a direct view of its history. Absorption spectra of lines of sight through the Milky Way and through nearby galaxies reveal a variety of chemical species, ionization conditions, and kinematic substructures. These absorption profiles are produced by low density gas distributed in rotating disks, high velocity halo clouds, satellite galaxies and their debris, superbubbles, and other sub-galactic gaseous fragments. Guided by knowledge gained by studying nearby galaxies, we are developing interpretations of the variety of observed absorption signatures. Images of .4 < z < 1 galaxies responsible for MgII absorption also allow us to explore the statistical connections between the galaxy properties and their gaseous content. Quasar absorption lines are fast becoming a powerful evolutionary probe of gaseous conditions in the Milky Way., Comment: 10 pages, Latex file, and 3 PostScript figures; To appear in Galactic Chemodynamics 4: The History of the Milky Way and its Satellite System, eds. A. Burkert, D. Hartmann, and S. Majewski (PASP Conference Series); Available as a single PostScript file at http://www.ucolick.org/~cwc/qso/abstract.html

We perform a double line of sight (DLOS) analysis of the Lyman-alpha 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=12.5kpc up to 800kpc, and a ``control sample'' of unrelated lines of sight, are analyzed at redshifts 3, 2, and 1. Coincident line samples are defined for HI column density thresholds of Nco = 10^{12.5}, 10^{13}, and 10^{14} per square cm. We find that: 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 N(HI) 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., Comment: 17 pages, Latex file, and 8 PostScript figures; Submitted to the Astrophysical Journal; Available as a single compressed Postscript file at http://www.astro.psu.edu/users/charlton

We challenge the conventional view that the majority of MgII/Lyman limit absorbers are extended halos of galaxies comprised of ``clouds'' with near-unity covering factor. The gaseous disks of spiral galaxies are known to extend to large radii and likely contribute a significant cross-section for absorption. We perform a Monte-Carlo survey of QSO fields in which the model galaxies have MgII absorbing ``clouds'' in a spherical halo or in a randomly oriented disk. For both geometries, models that recover the observed properties of MgII absorbers have only a 70-80% covering factor. Therefore, regardless of absorber geometry, a survey of randomly selected QSO fields should yield a non-negligible number of non-absorbing galaxies at small impact parameters. Very few have been observed (Steidel (1995)). However, selection effects are important, and once the observational procedures are applied to our model fields we find that this result is expected. Since both spherical halo and disk models can be made consistent with survey results, we present tests for discerning the geometric distribution of absorbing gas from Hubble Space Telescope images and high resolution spectra., Comment: 22 pages, Latex file, and 7 PostScript figures; Accepted to the Astrophysical Journal; Available as a single compressed PostScript file at http://www.ucolick.org/~cwc/qso/abstract.html

From R-band images of 42 Hickson compact groups, we present a sample of 47 candidate dwarf galaxies that are associated with the tidal tails and arms in the groups. The candidates, found in 15 tidal features, have R magnitudes and masses (for M/L = 1) in the ranges -16.5 < M_R - 5 log h_{75} < -11.5 and 2x10^6 M_{\odot} < M < 2x10^8 M_{\odot}, respectively. Their masses and locations are compared to the predictions of theoretical/N-body tidal dwarf formation scenarios. Considering the longevity of tidal debris in the compact group environment and the results of this survey, we estimate the contribution of the tidal dwarf formation mechanism to the population of dwarf galaxies observed at large in compact groups. If the majority of our dwarf galaxy candidates are confirmed as being gravitationally bound stellar systems, then a significant fraction, perhaps as much as one-half, of the dwarf population in compact groups is the product of interactions among giant parent galaxies., Comment: uuencoded gzipped Postscript files, 11 pages text + 4 images; also available at ftp://ftp.astro.psu.edu/pub/sdh ; to be published in ApJ

In this paper we explore the application of the pointwise dimension (PD) analysis as a large-scale structure descriptor to the RC3 catalog of galaxies (de Vaucouleurs et al. 1991). This technique, which originated in the field of fractal geometry (Mandelbrot 1983) and found many applications in non-linear dynamics, is particularly illuminating in the study of correlations between morphology and environment. To our knowledge the technique is being applied for the first time to the study of galaxy morphology and evolution. The PD is the slope of the curve representing the number of galaxies within some radius, determined separately for each galaxy in the catalog. Thus the distributions of PD's can be compared for subsamples based on morphological type or luminosity. The conclusions of this analysis of RC3 are: 1) The PD elucidates the tendency for early-type galaxies to cluster more than late-type galaxies; however, there is considerable overlap between these populations. 2) The PD statistic does not find a significant relationship between luminosity and clustering in RC3, but it could be an effective diagnostic in larger 2D catalogs. 3) The majority of galaxies in RC3 (excluding clusters) are spread out in space much like a random distribution. A population of objects that traces the distribution of galaxies, but avoids clusters, would have only small correlations. It is worthwhile to pursue application of the PD to larger catalogs with a wider range of luminosities., Comment: 13 pages uuencoded and compressed Postscript file. Full text, including figures, available at http://www.astro.psu.edu/users/best/Kingdom@/Academic@/fractal_paper.html or at ftp://ftp.astro.psu.edu/pub/best . To be published in January 1, 1996 issue of Astrophysical Journal

Study of Lyman-alpha absorption lines in the spectra of double line of sight (DLOS) quasars holds the promise of diagnosing the nature of the structures that give rise to Lyman-alpha absorption. Based on simulations of DLOS with various separations through a single population of absorbers, four tests have been designed to diagnose absorber mass distributions (smoothly varying with radius or irregular), geometries (spherical or disk/slab--like), and kinematics (isotropic or systematic velocities). Applying the tests to existing data at redshifts around two we find that: (1) The observed neutral hydrogen column density (N(HI)) distributions of Lyman-alpha lines coincident to both LOS are consistent with a smooth mass distribution. (2) Observed large anticoincident N(HI) are not consistent with a single population of smooth spherical absorbers, which should exhibit sharp cut--offs at small N(HI) in the N(HI) distribution. (3) There is marginal evidence that the observed RMS value of velocity differences between coincident lines increases with DLOS separation as is expected for disk/slab--like absorbers that have systematic velocity fields. (4) The observed velocity dispersion along a single LOS is small compared to the RMS difference between widely separated LOS, which is not consistent with models of cloudlets moving isotropically within a spherical structure. Overall, only a smooth disk/slab--like model with systematic velocities remains consistent with the inferred properties of a single population of Lyman-alpha absorbers., Comment: 7 pages with figures included, gzipped postscript; also available at http://ucowww.ucsc.edu/~cwc/qso/abstract.html ; Astrophysical Journal Letters (in press)

N-body simulations were conducted of pairs of galaxies with a 3:1 mass ratio on parabolic orbits, in order to quantify the effect of dynamical friction. The effects of varying the ratio of the dark matter halo size to the distance of closest approach were explored. Once the dark matter halos are fully overlapping, the more massive simulated galaxies achieve a larger maximum separation after the first encounter despite the increased dynamical friction caused by the more extended halos. Projected separation and radial velocity histograms were generated by "observing" the simulation results at various times and from various orientations. These histograms were compared with observations of galaxy pairs (Charlton & Salpeter 1991; Chengalur, Salpeter, & Terzian 1993) with the result that large halo radii (~200 - 600 kpc) and wide distances of closest approach are generally favored. It is difficult to reconcile the small radial velocity differences that have been observed (median of ~ 30 km/s, Chengalur et al. 1993) with the simulations when we sample all parts of the orbits equally. Including an additional population of wide pairs that have just recently reached "turnabout" from the Hubble flow would lower the median velocity differences. Models suggest that additional data for pairs at intermediate separations should have a somewhat larger median velocity difference than the wide pairs. Very narrow pairs include galaxies which are interacting, and whose gaseous components respond to forces other than gravity. If consistently small $\Delta v$ are measured from neutral hydrogen velocities in a larger sample of narrow pairs, pressure forces and dissipation effects on the gaseous components could be responsible., Comment: 16 pages, 10 figures, uuencoded gz-compressed tar file. To appear in the Astrophysical Journal.

The classic question regarding Lyman-alpha forest clouds is whether they are confined by the pressure of a hot, diffuse surrounding medium, or by the gravity of dark matter mini-halos. This paper reviews these basic models for forest clouds, considering spherical and slab geometries. At high redshift, the clouds are still likely to be in the process of formation, and it seems essential to consider them as dynamic structures in a cosmological context. At low redshift, observations of large cloud sizes (indicated by their covering of both lines of sight toward quasar pairs) have recently reshaped our view of the forest. Galaxy-like disks/slabs, which would be gravity confined near the center, but pressure confined in their outer regions (extending out to hundreds of kpcs), may be responsible for the low z forest. The ultimate view of the identity of the forest clouds and their relationship to galaxies is likely to involve a synthesis of many of the models discussed here., Comment: 10 pages and 7 figures, uuencoded gz-compressed tar file. To appear in the ESO QSO Absorption Line Workshop Proceedings}

The globular clusters that we observe in galaxies may be only a fraction of the initial population. Among the evolutionary influences on the population is the destruction of globular clusters by tidal forces as the cluster moves through the field of influence of a disk, a bulge, and/or a putative nuclear component (black hole). We have conducted a series of N-body simulations of globular clusters on bound and marginally bound orbits through potentials that include black hole and spheroidal components. The degree of concentration of the spheroidal component can have a considerable impact on the extent to which a globular cluster is disrupted. If half the mass of a $10^{10} \msun$ spheroid is concentrated within 800pc, then only black holes with masses $>10^9 \msun$ can have a significant tidal influence over that already exerted by the bulge. However, if the matter in the spheroidal component is not so strongly concentrated toward the center of the galaxy, a more modest central black hole (down to $10^8 \msun$) could have a dominant influence on the globular cluster distribution, particularly if many of the clusters were initially on highly radial orbits. Our simulations show that the stars that are stripped from a globular cluster follow orbits with roughly the same eccentricity as the initial cluster orbit, spreading out along the orbit like a ``string of pearls''. Since only clusters on close to radial orbits will suffer substantial disruption, the population of stripped stars will be on orbits of high eccentricity., Comment: 13 pages and 7 figures, uuencoded Z-compressed tar file. Accepted to The Astrophysical Journal

A break in the distribution function of Ly$\alpha$ clouds (at a typical redshift of $2.5$) has been reported by Petitjean et al. (1993). This feature is what would be expected from a transition between pressure confinement and gravity confinement (as predicted in Charlton, Salpeter, and Hogan (1993)). The column density at which the feature occurs has been used to determine the external confining pressure, $\sim 10 {\rm cm}^{-3} {\rm K}$, which could be due to a hot, intergalactic medium. For models that provide a good fit to the data, the contribution of the gas in clouds to $\Omega$ is small. The specific shape of the distribution function at the transition (predicted by models to have a non-monotonic slope) can serve as a diagnostic of the distribution of dark matter around Ly$\alpha$ forest clouds, and the present data already eliminate certain models., Comment: 10 pages plain TeX, 2 figures available upon request, submitted to ApJ Letters, PSU-jc-1