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Register a new userH_2-bearing damped Lyman-alpha systems as tracers of cosmological chemical evolution
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M. T. Murphy
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The chemical abundances in damped Lyman-alpha systems (DLAs) show more than 2 orders of magnitude variation at a given epoch, possibly because DLAs arise in a wide variety of host galaxies. This could significantly bias estimates of chemical evolution. We explore the possibility that DLAs in which H_2 absorption is detected may trace cosmological chemical evolution more reliably since they may comprise a narrower set of physical conditions. The 9 known H_2 absorption systems support this hypothesis: metallicity exhibits a faster, more well-defined evolution with redshift than in the general DLA population. The dust-depletion factor and, particularly, H_2 molecular fraction also show rapid increases with decreasing redshift. We comment on possible observational selection effects which may bias this evolution. Larger samples of H_2-bearing DLAs are clearly required and may constrain evolution of the UV background and DLA galaxy host type with redshift.
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We characterize the importance of metallicity on the presence of molecular hydrogen in damped Lyman-alpha (DLA) systems. We construct a representative sample of 18 DLA/sub-DLA systems with log N(HI)>19.5 at high redshift (zabs>1.8) with metallicities relative to solar [X/H]>-1.3(with[X/H]= logN(X)/N(H)-log(X/H)solar and X either Zn, S or Si). We gather data covering the expected wavelength range of redshifted H2 absorption lines on all systems in the sample from either the literature (10 DLAs), the UVES-archive or new VLT-UVES observations for four of them. The sample is large enough to discuss for the first time the importance of metallicity as a criterion for the presence of molecular hydrogen in the neutral phase at high-z. From the new observations, we report two new detections of molecular hydrogen in the systems at zabs=2.431 toward Q2343+125 and zabs=2.426 toward Q2348-011. We compare the H2 detection fraction in the high-metallicity sample with the detection fraction in the overall sample from Ledoux et al. (2003). We show that the fraction of DLA systems with logf=log 2N(H2)/(2N(H2)+N(HI))>-4 is as large as 50% for [X/H]>-0.7 when it is only about 5% for [X/H]<-1.3 and about 15% in the overall sample (with -2.5<[X/H]<-0.3). This demonstrates that the presence of molecular hydrogen at high redshift is strongly correlated with metallicity.
We have collected data for 69 Damped Lyman-alpha (DLA) systems, to investigate the chemical evolution of galaxies in the redshift interval 0.0 < z < 4.4. In doing that, we have adopted the most general approach used so far to correct for dust depletion. The best solution, obtained through chi^2 minimization, gives as output parameters the global DLA metallicity and the dust-to-metals ratio. Clear evolution of the metallicity vs. redshift is found (99.99% significance level), with average values going from ~1/30 solar at z~4.1 to ~3/5 solar at z~0.5. We also find that the majority of DLAs (~60%) shows dust depletion patterns which most closely resemble that of the warm halo clouds in the Milky Way, and have dust-to-metals ratios very close to warm halo clouds.
The XQ-100 survey has provided high signal-noise spectra of 100 redshift 3-4.5 quasars with the X-Shooter spectrograph. The metal abundances for 13 elements in the 41 damped Lyman alpha systems (DLAs) identified in the XQ-100 sample are presented, and an investigation into abundances of a variety of DLA classes is conducted. The XQ-100 DLA sample contains five DLAs within 5000 km/s of their host quasar (proximate DLAs; PDLAs) as well as three sightlines which contain two DLAs within 10,000 km/s of each other along the same line-of-sight (multiple DLAs; MDLAs). Combined with previous observations in the literature, we demonstrate that PDLAs with logN(HI)<21.0 show lower [S/H] and [Fe/H] (relative to intervening systems with similar redshift and N(HI)), whilst higher [S/H] and [Si/H] are seen in PDLAs with logN(HI)>21.0. These abundance discrepancies are independent of their line-of-sight velocity separation from the host quasar, and the velocity width of the metal lines (v90). Contrary to previous studies, MDLAs show no difference in [alpha/Fe] relative to single DLAs matched in metallicity and redshift. In addition, we present follow-up UVES data of J0034+1639, a sightline containing three DLAs, including a metal-poor DLA with [Fe/H]=-2.82 (the third lowest [Fe/H] in DLAs identified to date) at z=4.25. Lastly we study the dust-corrected [Zn/Fe], emphasizing that near-IR coverage of X-Shooter provides unprecedented access to MgII, CaII and TiII lines (at redshifts 3-4) to provide additional evidence for subsolar [Zn/Fe] ratio in DLAs.
We report evidence for a bimodality in damped Ly systems (DLAs). Using [C II] 158 mu cooling rates, lc, we find a distribution with peaks at lc=10^-27.4 and 10^-26.6 ergs s^-1 H^-1 separated by a trough at lc^crit ~= lc < 10^-27.0 ergs s^-1 H^-1. We divide the sample into low cool DLAs with lc < lc^crit and high cool DLAs with lc > lc^crit and find the Kolmogorv-Smirnov probabilities that velocity width, metallicity, dust-to-gas ratio, and Si II equivalent width in the two subsamples are drawn from the same parent population are small. All these quantities are significantly larger in the high cool population, while the H I column densities are indistinguishable in the two populations. We find that heating by X-ray and FUV background radiation is insufficient to balance the cooling rates of either population. Rather, the DLA gas is heated by local radiation fields. The rare appearance of faint, extended objects in the Hubble Ultra Deep Field rules out in situ star formation as the dominant star-formation mode for the high cool population, but is compatible with in situ star formation as the dominant mode for the low cool population. Star formation in the high cool DLAs likely arises in Lyman Break galaxies. We investigate whether these properties of DLAs are analogous to the bimodal properties of nearby galaxies. Using Si II equivalent width as a mass indicator, we construct bivariate distributions of metallicity, lc, and areal SFR versus the mass indicators. Tentative evidence is found for correlations and parallel sequences, which suggest similarities between DLAs and nearby galaxies. We suggest that the transition-mass model provides a plausible scenario for the bimodality we have found. As a result, the bimodality in current galaxies may have originated in DLAs.
(Abridged) We present the results of a systematic GBT and GMRT survey for 21-cm absorption in a sample of 10 DLAs at 2<z_abs<3.4. Analysis of L-band VLBA images of the background QSOs are also presented. We detect 21-cm absorption in only one DLA (at z_abs = 3.1745 towards J1337+3152). Combining our data with the data from the literature (a sample of 28 DLAs) and assuming the measured core fraction at milliarcsecond scale to represent the gas covering factor, we find that the HI gas in DLAs at z> 2 is predominantly constituted by WNM. The detection rate of 21-cm absorption seems to be higher for systems with higher N(HI) or metallicity. However, no clear correlation is found between the integrated 21-cm optical depth (or spin temperature) and either N(HI), metallicity or velocity spread of the low ionization species. There are 13 DLAs in our sample for which high resolution optical spectra covering the expected wavelength range of H_2 absorption are available. We report the detection of H_2 molecules in the z_abs = 3.3871 21-cm absorber towards J0203+1134 (PKS 0201+113). In 8 cases, neither H_2 nor 21-cm absorption are detected. The lack of 21-cm and H_2 absorption in these systems can be explained if most of the HI in these DLAs originate from low density high temperature gas. In one case we have a DLA with 21-cm absorption not showing H_2 absorption. In two cases, both species are detected but do not originate from the same velocity component. In the remaining 2 cases 21-cm absorption is not detected despite the presence of H_2 with evidence for the presence of cold gas. All this is consistent with the idea that the H_2 components seen in DLAs are compact (with sizes of < 15 pc) and contain only a small fraction (i.e typically <10%) of the total N(HI) measured in the DLAs.
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