Varsha P. Kulkarni

Average extinction curves and relative abundances for quasi-stellar object absorption-line systems at 1 ≤zabs < 2

We have studied a sample of 809 Mg II absorption systems with 1.0 ≤ z abs ≤ 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-A 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 A. 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 a 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 N H1 ∼ 10 20 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.

A Uniform Analysis of the Lyα Forest at z = 0-5. II. Measuring the Mean Intensity of the Extragalactic Ionizing Background Using the Proximity Effect

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 (~1 A) QSO spectra. These spectra were presented and the statistics of the Lyα forest were discussed in Paper I. In this analysis, we demonstrate that a proximity effect is present in the data; i.e., there exists a significant (5.5 σ) deficit of lines at zabs ≈ zem. 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(ν0) is found to be 1.4 × 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 [O III] 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 [O III] 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(ν0), 7.0 × 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 quasar luminosity function. Allowing for the fact that individual QSOs have different spectral indices that may also be different from that of the background, we use the standard methods to solve for the H I photoionization rate, Γ, and the parameters describing its evolution with redshift. The best-fit value for the H I ionization rate we derive is 1.9 × 10-12 s-1, in good agreement with models of the background that incorporate QSOs only. Finally, we use simulated Lyα 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 2-3. 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.

Hubble Space Telescope Observations of Element Abundances in Low-Redshift Damped Lyα Galaxies and Implications for the Global Metallicity-Redshift Relation

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 Lyα 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. 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) on board the Hubble Space Telescope (HST). These observations have provided the first constraints on Zn abundances in DLAs with z < 0.4. In all 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-weighted mean Zn metallicity versus 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 instead of the Zn limits. The corresponding estimates of the z = 0 intercept of the metallicity-redshift relation are -0.74 ± 0.15, -0.75 ± 0.18, and -0.71 ± 0.13, respectively. Roughly similar results are obtained if survival analysis or an unbinned N-weighted nonlinear χ2 approach is used. Thus, the N-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.

Metals and Dust in Intermediate-Redshift Damped Lyα Galaxies

We report on spectroscopic observations with the Multiple Mirror Telescope for 11 damped Lyα absorbers (DLAs) or strong DLA candidates at 0.1 < z < 1.5, including several absorbers discovered in the Sloan Digital Sky Survey. In particular, we have measured absorption lines of Zn II, Cr II, Ni II, Fe II, Mn II, Ti II, Ca II, and Si II. These measurements have doubled the sample of Zn and Cr measurements at z < 1. The average relative abundance patterns in these objects are very similar to those found for high-redshift DLAs reported in the literature. Our observations suggest that the dust content, as determined by [Cr/Zn], does not show much change with redshift. We also examine the sample for correlation of [Cr/Zn] with estimates of the quasar reddening. Our data suggest that the global mean metallicity of DLAs, as measured by the gas-phase abundance of Zn, at best shows a weak evolution with redshift over the range 0.4 < z < 3.9.

The Role of Sub-Damped Lyα Absorbers in the Cosmic Evolution of Metals

Observations of low mean metallicity of damped Lyα (DLA) quasar absorbers at all redshifts studied appear to contradict the predictions for the global mean interstellar metallicity in galaxies from cosmic chemical evolution models. On the other hand, a number of metal-rich sub-DLA systems have been identified recently, and the fraction of metal-rich sub-DLAs appears to be considerably larger than that of metal-rich DLAs, especially at z < 1.5. In view of this, here we investigate the evolution of metallicity in sub-DLAs. We find that the mean Zn metallicity of the observed sub-DLAs may be higher than that of the observed DLAs, especially at low redshifts, reaching a near-solar level at z 1. This trend does not appear to be an artifact of sample selection, the use of Zn, the use of NH i weighting, or observational sensitivity. While a bias against very low metallicity could be present in the sub-DLA sample in some situations, this cannot explain the difference between the DLA and sub-DLA metallicities at low z. The primary reason for the difference between the DLAs and sub-DLAs appears to be the dearth of metal-rich DLAs. We estimate the sub-DLA contribution to the total metal budget using measures of their metallicity and comoving gas density. These calculations suggest that at z 1, the contribution of sub-DLAs to the total metal budget may be several times that of DLAs. At higher redshifts also, there are indications that the sub-DLAs may contribute significantly to the cosmic metal budget.

A SINFONI integral field spectroscopy survey for galaxy counterparts to damped Lyman α systems – I. New detections and limits for intervening and associated absorbers★

Detailed studies of damped and sub-damped Lyman alpha systems (DLAs), the galaxies probed by the absorption they produce in the spectra of background quasars, rely on identifying the galaxy responsible for the absorber with more traditional methods. Integral field spectroscopy provides an efficient way of detecting faint galaxies near bright quasars, further providing immediate redshift confirmation. Here, we report the detection of H alpha emission from a DLA and a sub-DLA galaxy among a sample of six intervening quasar absorbers targeted. We derive F(H alpha) = 7.7 +/- 2.7 x 10(-17) erg s(-1) cm(-2) (SFR = 1.8 +/- 0.6M(circle dot) yr(-1)) at impact parameter b = 25 kpc towards quasar Q0302 - 223 for the DLA at z(abs) = 1.009 and F(H alpha) = 17.1 +/- 6.0 x 10(-17) erg s(-1) cm(-2) (SFR = 2.9 +/- 1.0M(circle dot) yr(-1)) at b = 39 kpc towards Q1009 - 0026 for the sub-DLA at z(abs) = 0.887. These results are in line with low star formation rates previously reported in the literature for quasar absorbers. We use the [N II]lambda 6585/H alpha ratio to derive the H II emission metallicities and compare them with the neutral gas HI absorption metallicities derived from high-resolution spectra. In one case, the absorption metallicity is actually found to be higher than the emission line metallicity. For the remaining objects, we achieve 3 sigma limiting fluxes of the order F(H alpha) similar to 10(-17) erg s(-1) cm(-2) (corresponding to SFR similar to 0.1 M-circle dot yr(-1) at z similar to 1 and similar to 1 M-circle dot yr(-1) at z similar to 2), i.e. among the lowest that have been possible with ground-based observations. We also present two other galaxies associated with C IV systems and serendipitously discovered in our data.

Metallicity Evolution of Damped Lyα Galaxies

We have reanalyzed the existing data on zinc abundances in damped Lyα (DLA) absorbers to investigate whether their mean metallicity evolves with time. Most models of cosmic chemical evolution predict that the mass-weighted mean interstellar metallicity of galaxies should rise with time from a low value of ~1/30 solar at z ~ 3 to a nearly solar value at z ~ 0. However, several previous analyses have suggested that there is little or no evolution in the global metallicity of DLAs. The main problem is that the effective number of systems that dominate the N(H I)-weighted mean metallicity is very small. We have used a variety of statistical techniques to quantify the global metallicity-redshift relation and its uncertainties, taking into account both measurement and sampling errors. Three new features of our analysis are (1) an unbinned N(H I)-weighted nonlinear χ2 fit to an exponential relation, (2) survival analysis to treat the large number of limits in the existing data, and (3) a comparison of the data with several models of cosmic chemical evolution based on an unbinned N(H I)-weighted χ2. We find that a wider range of evolutionary rates is allowed by the present data than claimed in previous studies. The slope of the exponential fit to the N(H I)-weighted mean Zn metallicity versus redshift relation is -0.20 ± 0.11 counting limits as detections and -0.27 ± 0.12 counting limits as zeros. Similar results are also obtained if the data are binned in redshift and if survival analysis is used. These slopes are marginally consistent with no evolution but are also consistent with the rates predicted by several models of cosmic chemical evolution (e.g., slopes of -0.61 to -0.25 for the models of Pei & Fall, Malaney & Chaboyer, and Pei et al.). The χ2 values obtained for most of these models are somewhat worse than that for the exponential model because the models lie above the observed data points, but still suggest that the present DLA data could indicate some evolution of the metallicity with redshift. Finally, we outline some future measurements necessary to improve the statistics of the global metallicity-redshift relation.

The proximity effect and the mean intensity of ionizing radiation at low redshifts

We report the first tentative detection of the proximity effect in the distribution of Lyα forest lines at low redshifts (z≤1). This is based on observations of 13 quasars by Bahcall et al. with the Faint Object Spectrograph on the Hubble Space Telescope as part of the Quasar Absorption Line Key Project. We estimate the mean intensity of ionizing radiation on the usual assumption that the proximity effect is caused entirely by the increased photoionization of absorbers near the quasars. The result is J L ∼6×10 −24 ergs s −1 cm −2 sr −1 Hz −1 at z≃0.5. However, the uncertainties are large, and even at the 1 σ level, J L could be lower by a factor of 3 or higher by a factor of 6

The most metal-rich intervening quasar absorber known

The metallicity in portions of high-redshift galaxies has been successfully measured thanks to the gas observed in absorption in the spectra of quasars, in the Damped Lyman-α systems (DLAs). Surprisingly, the global mean metallicity derived from DLAs is about 1/10th solar at 0 < z < 4 leading to the so-called “missing-metals problem”. In this paper, we present high-resolution observations of a sub-DLA system at zabs = 0.716 with super-solar metallicity toward SDSS J1323−0021. This is the highest metallicity intervening high-H i quasar absorber currently known, and is only the second super-solar such absorber known to date. We provide a detailed study of this unique object from VLT/UVES spectroscopy. We derive [Zn/H] =+ 0.61, [Fe/H] = −0.51, [Cr/H] = < −0.53, [Mn/H] = −0.37, and [Ti/H] = −0.61. Observations and photoionisation models using the CLOUDY software confirm that the gas in this sub-DLA is predominantly neutral and that the abundance pattern is probably significantly different from a Solar pattern. Fe/Zn and Ti/Zn vary among the main velocity components by factors of ∼ 3a nd ∼35, respectively, indicating non-uniform dust depletion. Mn/Fe is super-solar in almost all components, and varies by a factor of ∼3 among the dominant components. It would be interesting to observe more sub-DLA systems and determine whether they might contribute significantly toward the cosmic budget of metals.

Elemental abundance measurements in low-redshift damped Lyman α absorbers

ABSTRACT We present elemental abundance measurements for 9 damped Ly-α systems (DLAs) and 1sub-DLA at 0.1.z .1.5 from recent observations with the Multiple Mirror Telescope. Mostof these absorbers are found to be metal-poor, while 2 are found to have ≈ 30 − 50% solarmetallicities. Combining our data with other data from the literature, we find that the systemswith higher [Zn/H] also have stronger depletion as measured by [Cr/Zn] and [Fe/Zn]. The re-lationship between the metallicity and H I column density is also investigated. Together withour previous MMT survey (Khare et al. 2004) we have discovered 2 of the 4 known absorbersat z < 1.5 that lie above (although near) the “obscuration threshold” . This appears to be aresult of selecting absorbers with strong metal lines in our sample. It would be interesting tofind other similar systems by observing a larger sample and st udy how much such systemscontribute to the cosmic budget of metals. Finally, an analysis of the N HI -weighted meanmetallicity vs. redshift for our sample combined with data from the literature supports previ-ous conclusions that the N