Moderate resolution data for 40 quasi-stellar objects (QSOs) at z ≈ 2 were combined with spectra of comparable resolution of 59 QSOs with redshifts greater than 1.7 found in the literature to form a large, homogeneous sample of moderate resolution (∼1Å) QSO spectra. These spectra were presented and the statistics of the Lyman α forest were discussed in Paper I. In this analysis, we demonstrate that a proximity effect is present in the data, ie. there exists a significant (5.5σ) deficit of lines at z abs ≈ z em . Within 1.5 h −1 Mpc of the QSO emission redshift, the significance does depend on QSO luminosity, in accordance with the theory that this effect is caused by enhanced ionization of hydrogen in the vicinity of the QSO from UV photons from the QSO itself. The photoionization model of Bajtlik, Duncan, & Ostriker permits an estimate of the mean intensity of the extragalactic background radiation at the Lyman limit. We compare the results of this standard analysis with those obtained using a maximum likelihood technique. If the spectrum of the background is assumed to be identical to that of each individual QSO, and if this background is assumed to be constant over the redshift range 1.7 < z < 3.8, then the best fit value for J(ν 0 ) is found to be 1.4 +1.1 −0.5 × 10 −21 ergs s −1 cm −2 Hz −1 sr −1 , using QSO redshifts based on the Lyα emission line. Systemic QSO redshifts based on the [OIII] λ5007 emission line for 19 objects in our sample show an average redshift of ∼400 km s −1 with respect to Lyα emission. Using redshifts based on [OIII] or Mg II for the 35 objects for which they are measured and adding 400 km s −1 to the remaining QSO Lyα redshifts gives a lower value of J(ν 0 ), 7.0 +3.4 −4.4 × 10 −22 ergs s −1 cm −2 Hz −1 sr −1 . This value is in reasonable agreement with the predictions of various models of the ionizing background based on the integrated QSO luminosity function. Allowing for the fact that individual QSOs have different spectral indicies which may also be different from that of the background, we use the standard methods to solve for the HI photoionization rate, Γ, and the parameters describing its evolution with redshift. The best fit value for the HI ionization rate we derive is 1.9 +1.2 −1.0 × 10 −12 s −1 , in good agreement with models of the background which incorporate QSOs only. Finally, we use simulated Lyman α forest spectra including the proximity effect to investigate curve-of-growth effects in the photoionization model used in the analysis. We find that the presence of lines on the saturated part of the curve-of-growth could cause our estimates of the background intensity to be overestimated by a factor of two to three. This large absorption line sample and these techniques for measuring the background and understanding the systematics involved allow us to place what we believe are the firmest limits on the background at these redshifts.

Most models of cosmic chemical evolution predict that the mass-weighted mean interstellar metallicity of galaxies should rise with time from a low value ∼ 1/30 solar at z ∼ 3 to a nearly solar value at z = 0. In the absence of any selection effects, the damped Lyman-alpha absorbers (DLAs) in quasar spectra are expected to show such a rise in global metallicity. However, it has been difficult to determine whether or not DLAs show this effect, primarily because of the very small number of DLA metallicity measurements at low redshifts. -2 -In an attempt to put tighter constraints on the low-redshift end of the DLA metallicity-redshift relation, we have observed Zn II and Cr II lines in four DLAs at 0.09 < z < 0.52, using the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope (HST). These observations have provided the first constraints on Zn abundances in DLAs with z < 0.4. In all the three DLAs for which our observations offer meaningful constraints on the metallicity, the data suggest that the metallicities are much lower than the solar value. These results are consistent with recent imaging studies indicating that these DLAs may be associated with dwarf or low surface brightness galaxies.We combine our results with higher redshift data from the literature to estimate the global mean metallicity-redshift relation for DLAs. We find that the global mean metallicity shows at most a slow increase with decreasing redshift. For the redshift range 0.09 < z < 3.90, the slope of the exponential fit to the binned N(H I)-weighted mean Zn metallicity vs. redshift relation is −0.18 ± 0.06 counting Zn limits as detections, −0.22 ± 0.08 counting Zn limits as zeros, and −0.23 ± 0.06 using constraints on metallicity from other elements in cases of Zn limits. The corresponding estimates of the z = 0 intercept of the metallicityredshift relation are −0.74 ± 0.15, −0.75 ± 0.18, and −0.71 ± 0.13, respectively. Roughly similar results are obained if survival analysis or an unbinned N(H I)weighted nonlinear χ 2 approach is used. Thus, the N(H I)-weighted mean metallicity of DLAs does not appear to rise up to solar or near-solar values at z = 0. This weak evolution could be explained by the fact that our absorption-selected sample seems to be dominated by dwarf or low surface brightness galaxies. This suggests that current DLA samples, especially those at low redshifts, could be biased against more enriched galaxies because the latter may cause higher dust obscuration of the background quasars.

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We report three additional Spectrograph for Integral Field Observations in the Near Infrared (SINFONI) detections of Hα emission line from quasar absorbers, two of which are new identifications. These were targeted among a sample of systems with and metallicities measured from high‐resolution spectroscopy. The detected galaxies are at impact parameters ranging from 6 to 12 kpc from the quasar’s line of sight. We derive star formation rates (SFRs) of a few M⊙ yr−1 for the two absorbers at zabs∼ 1 and SFR = 17 M⊙ yr−1 for the damped Lyman α system (DLA) at zabs∼ 2. These three detections are found among a sample of 16 DLAs and sub‐DLAs (five at zabs∼ 1 and seven at zabs∼ 2). For the remaining undetected galaxies, we derive flux limits corresponding to SFR < 0.1–11.0 M⊙ yr−1 depending on redshift of the absorber and depth of the data. When combined with previous results from our survey for galaxy counterparts to H i‐selected absorbers, we find a higher probability of detecting systems with higher metallicity as traced by dust‐free [Zn/H] metallicity. We also report a higher detection rate with SINFONI for host galaxies at zabs∼ 1 than for systems at zabs∼ 2. Using the [N ii]/Hα ratio, we can thus compare absorption and emission metallicities in the same high‐redshift objects, more than doubling the number of systems for which such measures are possible.

We have studied a sample of 809 Mg ii absorption systems with 1.0 ≤zabs≤ 1.86 in the spectra of Sloan Digital Sky Survey quasi‐stellar objects (QSOs), with the aim of understanding the nature and abundance of the dust and the chemical abundances in the intervening absorbers. Normalized, composite spectra were derived, for abundance measurements, for the full sample and several subsamples, chosen on the basis of the line strengths and other absorber and QSO properties. Average extinction curves were obtained for the subsamples by comparing their geometric mean spectra with those of matching samples of QSOs without absorbers in their spectra. There is clear evidence for the presence of dust in the intervening absorbers. The 2175‐Å feature is not present in the extinction curves, for any of the subsamples. The extinction curves are similar to the Small Magellanic Cloud (SMC) extinction curve with a rising ultraviolet (UV) extinction below 2200 Å. The absorber rest‐frame colour excess, E(B−V), derived from the extinction curves, depends on the absorber properties and ranges from <0.001 to 0.085 for various subsamples. The column densities of Mg ii, Al ii, Si ii, Ca ii, Ti ii, Cr ii, Mn ii, Fe ii, Co ii, Ni ii and Zn ii do not show such a correspondingly large variation. The overall depletions in the high E(B−V) samples are consistent with those found for individual damped Lyman α systems, the depletion pattern being similar to halo clouds in the Galaxy. Assuming an SMC gas‐to‐dust ratio, we find a trend of increasing abundance with decreasing extinction; systems with NH I∼ 1020 cm−2 show solar abundance of Zn. The large velocity spread of strong Mg ii systems seems to be mimicked by weak lines of other elements. The ionization of the absorbers, in general appears to be low: the ratio of the column densities of Al iii to Al ii is always less than 1/2. QSOs with absorbers are, in general, at least three times as likely to have highly reddened spectra as compared to QSOs without any absorption systems in their spectra.