We present optical and near-infrared ground-based imaging results on four low-redshift damped Lyα (DLA) galaxies. The corresponding DLA systems were discovered in our Hubble Space Telescope spectroscopic surveys for DLA lines in known strong Mg II absorption-line systems toward the quasars B2 0827+243 (z DLA = 0.525), PKS 0952+179 (z DLA = 0.239), PKS 1127−145 (z DLA = 0.313), and PKS 1629+120 (z DLA = 0.532). Two of the four DLA galaxies have confirmed slit redshifts, one has a photometric redshift consistent with the absorption-line redshift, and the fourth identification is based on the galaxy's proximity to the quasar sight line. The DLA galaxies span a mixture of morphological types from patchy, irregular, and low surface brightness to spiral galaxies. The luminosities range from 0.02L * K to 1.2L * K . We also discovered several extremely red objects (EROs) in two of these fields and discuss the possibility that they are associated with the DLA galaxies. These observations add to the small but growing list of DLA galaxies at low redshift. At the present time, 14 DLA galaxies in the redshift range 0.05 z 1 have been studied. The distributions of DLA galaxy properties for these 14 cases are discussed and some important trends emerge. Low-luminosity dwarf galaxies with small impact parameters dominate this small sample. Also, four of the five highest column density systems, which dominate in the determination of the cosmological neutral gas mass density, arise in low surface brightness dwarf galaxies. Zwaan et al. have shown that only 15% of the neutral gas at the present epoch is contained in low surface brightness galaxies. Thus, if the low-redshift DLA galaxy trends hold up with larger samples, it would indicate that a different population of objects is responsible for the bulk of the neutral hydrogen gas in the universe at z ≈ 0.5.

The Circum-Galactic Medium (CGM) can be probed through the analysis of absorbing systems in the line-of-sight to bright background quasars. We present measurements of the metallicity of a new sample of 15 sub-damped Lyman-α absorbers (sub-DLAs, defined as absorbers with 19.0 < log N(H I) < 20.3) with redshift 0.584 z abs 3.104 from the ESO Ultra-Violet Echelle Spectrograph (UVES) Advanced Data Products Quasar Sample (EUADP). We combine these results with other measurements from the literature to produce a compilation of metallicity measurements for 92 sub-DLAs as well as a sample of 362 DLAs. We apply a multi-element analysis to quantify the amount of dust in these two classes of systems. We find that either the element depletion patterns in these systems differ from the Galactic depletion patterns or they have a different nucleosynthetic history than our own Galaxy. We propose a new method to derive the velocity width of absorption profiles, using the modeled Voigt profile features. The correlation between the velocity width ∆V 90 of the absorption profile and the metallicity is found to be tighter for DLAs than for sub-DLAs. We report hints of a bimodal distribution in the [Fe/H] metallicity of low redshift (z < 1.25) sub-DLAs, which is unseen at higher redshifts. This feature can be interpreted as a signature from the metal-poor, accreting gas and the metal-rich, outflowing gas, both being traced by sub-DLAs at low redshifts.

We present results from a search for galaxies that give rise to damped Lyman α (DLA), sub‐DLA and Lyman limit system (LLS) absorption at redshifts 0.1 ≲z≲ 1 in the spectra of background quasars. The sample was formed from a larger sample of strong Mg ii absorbers (Wλ27960≥ 0.3 Å) whose H i column densities were determined by measuring the Lyα line in Hubble Space Telescope ultraviolet spectra. Photometric redshifts, galaxy colours and proximity to the quasar sightline, in decreasing order of importance, were used to identify galaxies responsible for the absorption. Our sample includes 80 absorption systems for which the absorbing galaxies have been identified, of which 54 are presented here for the first time. In some cases a reasonable identification for the absorbing galaxy could not be made. The main results of this study are (i) the surface density of galaxies falls off exponentially with increasing impact parameter, b, from the quasar sightline relative to a constant background of galaxies, with an e‐folding length of ≈46 kpc. Galaxies with b≳ 100 kpc calculated at the absorption redshift are statistically consistent with being unrelated to the absorption system, and are either background or foreground galaxies. (ii) is inversely correlated with b at the 3.0σ level of significance. DLA galaxies are found systematically closer to the quasar sightline, by a factor of 2, than are galaxies which give rise to sub‐DLAs or LLSs. The median impact parameter is 17.4 kpc for the DLA galaxy sample, 33.3 kpc for the sub‐DLA sample and 36.4 kpc for the LLS sample. We also find that the decline in with b can be roughly described by an exponential with an e‐folding length of 12 kpc that occurs at . (iii) Absorber galaxy luminosity relative to L*, L/L*, is not significantly correlated with Wλ27960, or b. (iv) DLA, sub‐DLA and LLS galaxies comprise a mix of spectral types, but are inferred to be predominantly late‐type galaxies based on their spectral energy distributions. (v) The properties of low‐redshift DLAs and sub‐DLAs are very different in comparison to the properties of gas‐rich galaxies at the present epoch. A significantly higher fraction of low‐redshift absorbers have large b values, and a significantly higher fraction of the large b value galaxies have luminosities L < L*. The implications of these results are discussed.

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We derive the statistical properties of neutral gas at redshifts 0.11 < z < 1.65 from UV measurements of quasar Lyα absorption lines corresponding to 369 Mg ii systems with W λ2796 0 ≥ 0.3Å. In addition to the 41 damped Lyman alpha (DLA) systems presented in Rao et al. (2006), the current DLA sample includes 29 newly discovered DLAs. Of these, 26 were found in our Hubble Space Telescope (HST) Advanced Camera for Surveys prism survey for DLAs (Turnshek et al. 2015) and three were found in a GALaxy Evolution Explorer (GALEX) archival search. In addition, an HST Cosmic Origins Spectrograph Cycle 19 survey yielded no DLAs that could be used for this study. Formally, this DLA sample includes 70 systems with N HI ≥ 2 × 10 20 atoms cm −2 . We find that the incidence of DLAs, or the product of their gas cross section and their comoving number density, can be described by n DLA (z) = (0.027±0.007)(1+z) (1.682±0.200) over the redshift range 0 < z < 5. The cosmic mass density of neutral gas can be described by Ω DLA (z) = (4.77 ± 1.60) × 10 −4 (1 + z) (0.64±0.27) . The low-redshift column density distribution function is well-fitted by a power law of the form f (N) ∼ N β with β = −1.46 ± 0.20. It is consistent with the high-redshift as well as z = 0 estimates at the high column density end but, lies between them at the low column density end. We discuss possible N HI and metallicity bias in Mg ii-selected DLA samples and show that such biases do not exist in the current data at z < 1.65. Thus, at least at z < 1.65, DLAs found through Mg ii selection statistically represent the true population of DLAs. However, we caution that studies of DLA metallicities should take into the account the relative incidence of DLAs with respect to W λ2796 0 (or gas velocity spread) in order to correctly measure the mean neutral-gas cosmic metallicity of the universe.