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Register a new userMetal abundances at z<1.5: new measurements in sub-Damped Lyman-alpha Absorbers
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C. Peroux
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Damped Lyman-alpha systems (DLAs) and sub-DLAs seen toward background quasars provide the most detailed probes of elemental abundances. Somewhat paradoxically these measurements are more difficult at lower redshifts due to the atmospheric cut-off, and so a few years ago our group began a programme to study abundances at z < 1.5 in quasar absorbers. In this paper, we present new UVES observations of six additional quasar absorption line systems at z < 1.5, five of which are sub-DLAs. We find solar or above solar metallicity, as measured by the abundance of zinc, assumed not to be affected by dust, in two sub-DLAs: one, towards Q0138-0005 with [Zn/H]=+0.28 +/- 0.16; the other towards Q2335+1501 with [Zn/H]=+0.07 +/- 0.34. Relatively high metallicity was observed in another system: Q0123-0058 with [Zn/H]=-0.45 +/- 0.20. Only for the one DLA in our sample, in Q0449-1645, do we find a low metallicity, [Zn/H]=-0.96 +/- 0.08. We also note that in some of these systems large relative abundance variations from component to component are observed in Si, Mn, Cr and Zn.
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Damped Lyman-alpha absorbers (DLAs), seen in absorption against a background quasar, provide the most detailed probes available of element abundances in the Universe over > 90 % of its age. DLAs can be used to observationally measure the global mean metallicity in the Universe and its evolution with time. Paradoxically, these observations are more difficult at lower redshifts, where the absorber rest-frame UV spectra are cut-off due to the atmospheric absorption. We present here high-resolution VLT/UVES observations of several elements contained in three DLAs and one sub-DLA with 0.6<z_abs<0.9. We detect Mg I, Mg II, Fe II, Zn II, Cr II, Mn II, Ti II and Ca II. Our observations more than double the high-resolution sample of [Zn/H] at z<1. We also report the discovery of three metal-rich systems, whereas most previous measurements show low N(HI)-weighted mean metallicity projecting to about 1/6th solar level at z=0. We derive [Zn/H]=-0.11+/-0.04 at z_abs=0.725, [Zn/H]=-0.54+/-0.20 at z_abs=0.740 and [Zn/H]=-0.49+/-0.22 at z_abs=0.652, plus one additional upper limit ([Zn/H]<-0.36 at z_abs=0.842). These measurements confirm the existence of quasar absorbers with relatively high metallicities based on abundance estimates free from the effect of dust depletion. Possible implications of these results for the metallicity of neutral gas phase in the past ~ 8 Gyr are presented and compared with models.
We present HIRES observations for 30 damped Lyman alpha systems, selected on the basis of their large metal column densities from previous, lower resolution data. The measured metal column densities for Fe, Zn, S, Si, Cr, Mn, and Ni are provided for these 30 systems. Combined with previously observed large metal column density damped Lyman alpha systems, we present a sample of 44 damped Lyman alpha systems observed with high resolution spectrographs (R~30000). These damped Lyman alpha systems probe the most chemically evolved systems at redshifts greater than 1.5. We discuss the context of our sample with the general damped Lyman alpha population, demonstrating that we are probing the top 10% of metal column densities with our sample. In a companion paper, we will present an analysis of the samples elemental abundances in the context of galactic chemical enrichment.
We study metal abundances in the z=0.9313 damped Lya system observed in the two lines-of-sight, A and B, toward the gravitationally-lensed double QSO HE0512-3329. Spatially resolved STIS spectra constrain the neutral-gas column density to be LogN(HI)=20.5 in both Aand B. UVES spectra (spectral resolution FWHM=9.8 km/s) show, in contrast, significant line-of-sight differences in the column densities of MnII and FeII; these are not due to observational systematics. We find that [Mn/H]=-1.44 and [Fe/H]=-1.52 in damped Lya system A, while [Mn/H]=-0.98 and [Fe/H]>-1.32, and possibly as high as [Fe/H] approx. -1 in damped Lya system B. A careful assessment of possible systematic errors leads us to conclude that these transverse differences are significant at a 5 sigma level or greater. Although nucleosynthesis effects may also be at play, we favor differential dust-depletion as the main mechanism producing the observed abundance gradient. The transverse separation is 5 kpc at the redshift of the absorber, which is also likely to be the lensing galaxy. The derived abundances therefore probe two opposite sides of a single galaxy hosting both damped Lya systems. This is the first time firm abundance constraints have been obtained for a single damped system probed by two lines-of-sight. The significance of this finding for the cosmic evolution of metals is discussed.
We consider the questions of whether the damped Lyman-alpha (DLA) and sub-DLA absorbers in quasar spectra differ intrinsically in metallicity, and whether they could arise in galaxies of different masses. Using the recent measurements of the robust metallicity indicators Zn and S in DLAs and sub-DLAs, we confirm that sub-DLAs have higher mean metallicities than DLAs, especially at $z lesssim 2$. We find that the intercept of the metallicity-redshift relation derived from Zn and S is higher than that derived from Fe by 0.5-0.6 dex. We also show that, while there is a correlation between the metallicity and the rest equivalent width of Mg II $lambda 2796$ or Fe II $lambda 2599$ for DLAs, no correlation is seen for sub-DLAs. Given this, and the similar Mg II or Fe II selection criteria employed in the discovery of both types of systems at lower redshifts, the difference between metallicities of DLAs and sub-DLAs appears to be real and not an artefact of selection. This conclusion is supported by our simulations of Mg II $lambda 2796$ and Fe II $lambda 2599$ lines for a wide range of physical conditions. On examining the velocity spreads of the absorbers, we find that sub-DLAs show somewhat higher mean and median velocity spreads ($Delta v$), and an excess of systems with $Delta v > 150$ km s$^{-1}$, than DLAs. Compared to DLAs, the [Mn/Fe] vs. [Zn/H] trend for sub-DLAs appears to be steeper and closer to the trend for Galactic bulge and thick disk stars, possibly suggesting different stellar populations. The absorber data appear to be consistent with galaxy down-sizing. The data are also consistent with the relative number densities of low-mass and high-mass galaxies. It is thus plausible that sub-DLAs arise in more massive galaxies on average than DLAs.
Nitrogen is thought to have both primary and secondary origins depending on whether the seed carbon and oxygen are produced by the star itself (primary) or already present in the interstellar medium (secondary) from which star forms. DLA and sub-DLA systems with typical metallicities of -3.0<Z/Z_sun<-0.5 are excellent tools to study nitrogen production. We made a search for nitrogen in the ESO-UVES advanced data products (EUADP) database. In the EUADP database, we find 10 new measurements and 9 upper limits of nitrogen. We further compiled DLA/sub-DLA data from the literature with estimates available of nitrogen and alpha-elements. This yields a total of 98 systems, i.e. the largest nitrogen abundance sample investigated so far. In agreement with previous studies, we indeed find a bimodal [N/alpha] behaviour: three-quarter systems show a mean value of [N/alpha]=-0.87 with a scatter of 0.21 dex and one-quarter shows ratios clustered at [N/alpha]=-1.43 with a lower dispersion of 0.13 dex. The high [N/alpha] group is consistent with the blue compact dwarves and dwarf irregular galaxies, suggesting primary nitrogen production. The low [N/alpha] group is the lowest ever observed in any astrophysical site and probably provides an evidence of the primary production by fast rotating massive stars in young sites. Moreover, we find a transition between the two [N/alpha] groups around [N/H]=-2.5. The transition is not abrupt and there are a few systems lying in the transition region. Additional observations of DLAs/sub-DLAs below [N/H]<-2.5 would provide more clues.
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