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Register a new userNew constraints on the physical conditions in H$_2$-bearing GRB-host damped Lyman-$alpha$ absorbers
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We report the detections of molecular hydrogen (H$_2$), vibrationally-excited H$_2$ (H$^*_2$), and neutral atomic carbon (CI), in two new afterglow spectra of GRBs,181020A ($z=2.938$) and 190114A ($z=3.376$), observed with X-shooter at the Very Large Telescope (VLT). Both host-galaxy absorption systems are characterized by strong damped Lyman-$alpha$ absorbers (DLAs) and substantial amounts of molecular hydrogen with $log N$(HI, H$_2$) = $22.20pm 0.05,~20.40pm 0.04$ (GRB,181020A) and $log N$(HI, H$_2$) = $22.15pm 0.05,~19.44pm 0.04$ (GRB,190114A). The DLA metallicites, depletion levels and dust extinctions are [Zn/H] = $-1.57pm 0.06$, [Zn/Fe] = $0.67pm 0.03$, and $A_V = 0.27pm 0.02$,mag (GRB,181020A) and [Zn/H] = $-1.23pm 0.07$, [Zn/Fe] = $1.06pm 0.08$, and $A_V = 0.36pm 0.02$,mag (GRB,190114A). We then examine the molecular gas content of all known H$_2$-bearing GRB-DLAs and explore the physical conditions and characteristics of these systems. We confirm that H$_2$ is detected in all CI- and H$^*_2$-bearing GRB absorption systems, but that these rarer features are not necessarily detected in all GRB H$_2$ absorbers. We find that a large molecular fraction of $f_{rm H_2} gtrsim 10^{-3}$ is required for CI to be detected. The defining characteristic for H$^*_2$ to be present is less clear, though a large H$_2$ column density is an essential factor. We then derive the H$_2$ excitation temperatures of the molecular gas and find that they are relatively low with $T_{rm ex} approx 100 - 300$,K, however, there could be evidence of warmer components populating the high-$J$ H$_2$ levels in GRBs,181020A and 190114A. Finally, we demonstrate that the otherwise successful X-shooter GRB afterglow campaign is hampered by a significant dust bias excluding the most dust-obscured H$_2$ absorbers from identification [Abridged].
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We report Keck/ESI and VLT/UVES observations of three super-damped Lyman-alpha quasar absorbers with H I column densities log N(HI) >= 21.7 at redshifts z=2-2.5. All three absorbers show similar metallicities (-1.3 to -1.5 dex), and dust depletion of Fe, Ni, and Mn. Two of the absorbers show supersolar [S/Zn] and [Si/Zn]. We combine our results with those for other DLAs to examine trends between N(HI), metallicity, dust depletion. A larger fraction of the super-DLAs lie close to or above the line [X/H]=20.59-log N(HI) in the metallicity vs. N(HI) plot, compared to the less gas-rich DLAs, suggesting that super-DLAs are more likely to be rich in molecules. Unfortunately, our data for Q0230-0334 and Q0743+1421 do not cover H2 absorption lines. For Q1418+0718, some H2 lines are covered, but not detected. CO is not detected in any of our absorbers. For DLAs with log N(HI) < 21.7, we confirm strong correlation between metallicity and Fe depletion, and find a correlation between metallicity and Si depletion. For super-DLAs, these correlations are weaker or absent. The absorbers toward Q0230-0334 and Q1418+0718 show potential detections of weak Ly-alpha emission, implying star formation rates of about 1.6 and 0.7 solar masses per year, respectively (ignoring dust extinction). Upper limits on the electron densities from C II*/C II or Si II*/Si II are low, but are higher than the median values in less gas-rich DLAs. Finally, systems with log N(HI) > 21.7 may have somewhat narrower velocity dispersions delta v_90 than the less gas-rich DLAs, and may arise in cooler and/or less turbulent gas.
We compute the z = 3 neutral hydrogen column density distribution function f(NHI) for 19 simulations drawn from the OWLS project using a post-processing correction for self-shielding calculated with full radiative transfer of the ionising background radiation. We investigate how different physical processes and parameters affect the abundance of Lyman-limit systems (LLSs) and damped Lyman-alpha absorbers (DLAs) including: i) metal-line cooling; ii) the efficiency of feedback from SNe and AGN; iii) the effective equation of state for the ISM; iv) cosmological parameters; v) the assumed star formation law and; vi) the timing of hydrogen reionization . We find that the normalisation and slope, D = d log10 f /d log10 NHI, of f(NHI) in the LLS regime are robust to changes in these physical processes. Among physically plausible models, f(NHI) varies by less than 0.2 dex and D varies by less than 0.18 for LLSs. This is primarily due to the fact that these uncertain physical processes mostly affect star-forming gas which contributes less than 10% to f(NHI) in the the LLS column density range. At higher column densities, variations in f(NHI) become larger (approximately 0.5 dex at NHI = 10^22 cm^-2 and 1.0 dex at NHI = 10^23 cm^-2) and molecular hydrogen formation also becomes important. Many of these changes can be explained in the context of self-regulated star formation in which the amount of star forming gas in a galaxy will adjust such that outflows driven by feedback balance inflows due to accretion. Data and code to reproduce all figures can be found at the following url: https://bitbucket.org/galtay/hi-cddf-owls-1
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.
We report Hubble Space Telescope Cosmic Origins Spectrograph far-ultraviolet and Arecibo Telescope H{sc i} 21cm spectroscopic studies of six damped and sub-damped Lyman-$alpha$ absorbers (DLAs and sub-DLAs, respectively) at $z lesssim 0.1$, that have yielded estimates of their H{sc i} column density, metallicity and atomic gas mass. This significantly increases the number of DLAs with gas mass estimates, allowing the first comparison between the gas masses of DLAs and local galaxies. Including three absorbers from the literature, we obtain H{sc i} masses $approx (0.24 - 5.2) times 10^9 : {rm M}_odot$, lower than the knee of the local H{sc i} mass function. This implies that massive galaxies do not dominate the absorption cross-section for low-$z$ DLAs. We use Sloan Digital Sky Survey photometry and spectroscopy to identify the likely hosts of four absorbers, obtaining low stellar masses, $approx 10^7-10^{8.7} M_odot$, in all cases, consistent with the hosts being dwarf galaxies. We obtain high H{sc i} 21,cm or CO emission line widths, $Delta V_{20} approx 100-290$~km~s$^{-1}$, and high gas fractions, $f_{rm HI} approx 5-100$, suggesting that the absorber hosts are gas-rich galaxies with low star formation efficiencies. However, the H{sc i} 21,cm velocity spreads ($gtrsim 100$~km~s$^{-1}$) appear systematically larger than the velocity spreads in typical dwarf galaxies.
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 data 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.
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