Jane C. Charlton

The Population of Weak Mg II Absorbers. I. A Survey of 26 QSO HIRES/Keck Spectra* **

We present a search for weak Mg II absorbers [those with Wr(2796) (2796)=0.02 A and that this is primarily a high-metallicity selection effect ([Z/Z☉] ≥ -1). This implies that Mg II absorbing structures figure prominently as tracers of sub-LLS environments where gas has been processed by stars. We compare the number density of Wr(2796) ≥ 0.02 A absorbers with that of both high and low surface brightness galaxies and find a fiducial absorber size of 35 h-1-63 h-1 kpc, depending upon the assumed galaxy population and their absorption properties. The individual absorbing clouds have Wr(2796) ≤ 0.15 A, and their narrow (often unresolved) line widths imply temperatures of ~25,000 K. We measured Wr(1548) from C IV in Faint Object Spectrograph/Hubble Space Telescope archival spectra and, based upon comparisons with Fe II, found a range of ionization conditions (low, high, and multiphase) in absorbers selected by weak Mg II.

The formation of dwarf galaxies in tidal debris: A study of the compact group environment

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.

The Population of Weak Mg II Absorbers. II. The Properties of Single-Cloud Systems

We present an investigation of Mg II absorbers characterized as single-cloud weak systems [defined by Wr(λ2796) < 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, Lyα, 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:

Low- and High-Ionization Absorption Properties of Mg II Absorption-selected Galaxies at Intermediate Redshifts. II. Taxonomy, Kinematics, and Galaxies* **

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. Lyα 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(Lyα) (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 Lyα/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 σ) 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.

Low and high ionization absorption properties of mg II absorption-selected galaxies at intermediate redshifts. I. general properties

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-1 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-1) Faint Object Spectrograph data (1600-3275 ?) 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.

High-Redshift Superwinds as the Source of the Strongest Mg II Absorbers: A Feasibility Analysis

We present High Resolution Echelle Spectrometer/Keck profiles of four extremely strong (Wr > 1.8 A) Mg II absorbers at 1 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 Lyα 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.

On the Origin of Intrinsic Narrow Absorption Lines in z ≲ 1 QSOs

We present an exhaustive statistical analysis of the associated (Δvabs -0.5], and mediocre C IV FWHM (6000 km s-1) do not have detectable associated NALs, down to Wr(C ) = 0.35 A. 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 A] 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.

The Kinematic Composition of Mg II Absorbers

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.

From globular clusters to tidal dwarfs: Structure formation in the tidal tails of merging galaxies

Using V and I images obtained with the Wide Field Planetary Camera 2 (WFPC2) of the Hubble Space Telescope, we investigate compact stellar structures within tidal tails. Six regions of tidal debris in the four classic Toomre sequence mergers: NGC 4038/39 (Antennae), NGC 3256, NGC 3921, and NGC 7252 (Atoms for Peace) have been studied in order to explore how the star formation depends on the local and global physical conditions. These mergers sample a range of stages in the evolutionary sequence and tails with and without embedded tidal dwarf galaxies. The six tails are found to contain a variety of stellar structures, with sizes ranging from those of globular clusters up to those of dwarf galaxies. From V and I WFPC2 images, we measure the luminosities and colors of the star clusters. NGC 3256 is found to have a large population of blue clusters (0.2 V-I 0.9), particularly in its western tail, similar to those found in the inner region of the merger. In contrast, NGC 4038/39 has no clusters in the observed region of the tail, only less luminous point sources likely to be individual stars. NGC 3921 and NGC 7252 have small populations of clusters along their tails. A significant cluster population is clearly associated with the prominent tidal dwarf candidates in the eastern and western tails of NGC 7252. The cluster-rich western tail of NGC 3256 is not distinguished from the others by its dynamical age or by its total H I mass. However, the mergers that have few clusters in the tail all have tidal dwarf galaxies, while NGC 3256 does not have prominent tidal dwarfs. We speculate that star formation in tidal tails may manifest itself either in small structures like clusters along the tail or in large structures such as dwarf galaxies, but not in both. Also, NGC 3256 has the highest star formation rate of the four mergers studied, which may contribute to the high number of star clusters in its tidal tails.

A Census of Intrinsic Narrow Absorption Lines in the Spectra of Quasars at z = 2-4*

We use Keck HIRES spectra of 37 optically bright quasars at z = 2-4 to study narrow absorption lines that are intrinsic to the quasars (intrinsic NALs, produced in gas that is physically associated with the quasar central engine). We identify 150 NAL systems, which contain 124 C IV, 12 N V, and 50 Si IV doublets, of which 18 are associated systems (within 5000 km s-1 of the quasar redshift). We use partial coverage analysis to separate intrinsic NALs from NALs produced in cosmologically intervening structures. We find 39 candidate intrinsic systems (28 reliable determinations and 11 that are possibly intrinsic). We estimate that 10%-17% of C IV systems at blueshifts of 5000-70,000 km s-1 relative to quasars are intrinsic. At least 32% of quasars contain one or more intrinsic C IV NALs. Considering N V and Si IV doublets showing partial coverage as well, at least 50% of quasars host intrinsic NALs. This result constrains the solid angle subtended by the absorbers to the background source(s). We identify two families of intrinsic NAL systems, those with strong N V absorption and those with negligible absorption in N V but with partial coverage in the C IV doublet. We discuss the idea that these two families represent different regions or conditions in accretion disk winds. Of the 26 intrinsic C IV NAL systems, 13 have detectable low-ionization absorption lines at similar velocities, suggesting that these are two-phase structures in the wind rather than absorbers in the host galaxy. We also compare possible models for quasar outflows, including radiatively accelerated disk-driven winds, magnetocentrifugally accelerated winds, and pressure-driven winds, and we discuss ways of distinguishing between these models observationally.