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تسجيل مستخدم جديدThe star formation rate of CaII and damped Lyman-alpha absorbers at 0.4<z<1.3
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[abridged] Using stacked Sloan Digital Sky Survey spectra, we present the detection of [OII]3727,3730 nebular emission from galaxies hosting CaII and MgII absorption line systems. Both samples of absorbers, 345 CaII systems and 3461 MgII systems, span the redshift interval 0.4 < z < 1.3; all of the former and half the latter sample are expected to be bona-fide damped Lyman-alpha (DLA) absorbers. The measured star formation rate (SFR) per absorber from light falling within the SDSS fibre apertures (corresponding to physical radii of 6-9 h^-1 kpc) is 0.11-0.14 Msol/yr for the MgII-selected DLAs and 0.11-0.48 Msol/yr for the CaII absorbers. These results represent the first estimates of the average SFR in an absorption-selected galaxy population from the direct detection of nebular emission. Adopting the currently favoured model in which DLAs are large, with radii >9h^-1 kpc, and assuming no attenuation by dust, leads to the conclusion that the SFR per unit area of MgII-selected DLAs falls an order of magnitude below the predictions of the Schmidt law, which relates the SFR to the HI column density at z~0. The contribution of both DLA and CaII absorbers to the total observed star formation rate density in the redshift range 0.4 < z < 1.3, is small, <10% and <3% respectively. The result contrasts with the conclusions of Hopkins et al. that DLA absorbers can account for the majority of the total observed SFR density in the same redshift range. Our results effectively rule out a picture in which DLA absorbers are the sites in which a large fraction of the total SFR density at redshifts z < 1 occurs.
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Absorption line studies of galaxies along the line-of-sight to distant quasars allow a direct observational link between the properties of the extended gaseous disk/halo and of the star forming region of galaxies. In these proceedings we review recen
t work on CaII absorbers detected in the SDSS at 0.4<z<1.3 which, because of their dust content and chemical properties, may lie spatially closer to the central host galaxy than most DLAs. We present direct evidence for the presence of star formation, through observation of the [OII]3727,3730 emission line, in both CaII absorbers and MgII-selected Damped Lyman-alpha (DLA) systems. The measured star formation rate (SFR) from light falling within the SDSS fibre apertures (corresponding to physical radii of 6-9 h^{-1}kpc) is 0.11-0.48 Msol/yr for the CaII-absorbers and 0.11-0.14 Msol/yr for the MgII-selected DLAs. The contribution of both CaII absorbers and DLAs to the total observed SFR density, in the redshift range 0.4 < z < 1.3, is small, <10%. Our result contrasts with recent conclusions, based on the Schmidt law, that DLA absorbers can account for the majority of the total observed SFR density in the same redshift range.
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 have obtained high signal:to:noise optical spectroscopy at 5AA resolution of 27 quasars from the APM z$>$4 quasar survey. The spectra have been analyzed to create new samples of high redshift Lyman-limit and damped Lyman-$alpha$ absorbers. These d
ata have been combined with published data sets in a study of the redshift evolution and the column density distribution function for absorbers with $log$N(HI)$ge17.5$, over the redshift range 0.01 $<$ z $<$ 5. The main results are: begin{itemize} item Lyman limit systems: The data are well fit by a power law $N(z) = N_0(1 + z)^{gamma}$ for the number density per unit redshift. For the first time intrinsic evolution is detected in the product of the absorption cross-section and comoving spatial number density for an $Omega = 1$ Universe. We find $gamma = 1.55$ ($gamma = 0.5$ for no evolution) and $N_0 = 0.27$ with $>$99.7% confidence limits for $gamma$ of 0.82 & 2.37. item Damped lya systems: The APM QSOs provide a substantial increase in the redshift path available for damped surveys for $z>3$. Eleven candidate and three confirmed damped Ly$alpha$ absorption systems, have been identified in the APM QSO spectra covering the redshift range $2.8le z le 4.4$ (11 with $z>3.5$). Combining the APM survey confirmed and candidate damped lya absorbers with previous surveys, we find evidence for a turnover at z$sim$3 or a flattening at z$sim$2 in the cosmological mass density of neutral gas, $Omega_g$. end{itemize} The Lyman limit survey results are published in Storrie-Lombardi, et~al., 1994, ApJ, 427, L13. Here we describe the results for the DLA population of absorbers.
The Damped Lyman-alpha systems (DLAs), seen in absorption in the spectrum of quasars, are believed to contain a large fraction of the neutral gas in the Universe. Paradoxically, these systems are more difficult to observe at z_abs<1.7, since they are
rare and their HI feature then falls in UV spectra. Rao & Turnshek (2000) pioneered a method based on MgII-selected DLAs, that is absorbers discovered thanks to our knowledge of their MgII feature in optical spectra. We use new observations undertaken at the TNG as well as a careful literature & archival search to build samples of low redshift absorbers classified according to the technique used for their discovery. We successfully recover N(HI) and equivalent widths of FeII 2600, MgII 2796, MgII 2803 and MgII 2852 for a sample of 36 absorbers, 21 of which are MgII-selected. We find that the MgII-selected sample contains a marginally larger fraction of absorbers with log N(HI)>21.0 than seen otherwise at low redshift. If confirmed, this property will in turn affect estimates of Omega_HI which is dominated by the highest HI column densities. We find that log N(HI) does not correlate significantly with metal equivalent widths. Similarly, we find no evidence that gravitational lensing, the fraction of associated systems or redshift evolution affect the absorber samples in a different way. We conclude that the hint of discrepancies in N(HI) distributions most likely arises from small number statistics. Therefore, further observations are required to better clarify the impact of this discrepancy on estimates of Omega_HI at low redshift.
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, an
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