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Register a new userPhysical conditions and redshift evolution of optically thin C III absorbers: Low-z sample
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We present a detailed analysis of 99 optically thin C III absorption systems at redshift, $0.2 le z le 0.9$ associated with neutral hydrogen column densities in the range, $15 le {rm log}$ $N_{rm H,I}$ ($cm^{-2}$) $le 16.2$. Using photoionization models, we infer the number density ($n_{rm H}$), C-abundance ($[C/H]$) and line-of-sight thickness ($L$) of these systems in the ranges, $-3.4 le$ log $n_{rm H}$ (in $cm^{-3}$) $le -1.6$, $-1.6 le [C/H] le 0.4$, and 1.3 pc $le L le$ 10 kpc, respectively with most of the systems having sub-kpc scale thickness. We combine the low$-z$ and previously reported high$-z$ ($2.1le zle 3.3$) optically thin C III systems to study the redshift evolution and various correlation between the derived physical parameters. We see a significant redshift evolution in $n_{rm H}$, $[C/H]$ and $L$. We compare the redshift evolution of metallicity in C III systems with those of various types of absorption systems. We find that the slope of $[C/H]$ vs. $z$ for C III absorbers is stepper compared to the redshift evolution of cosmic metallicity of the damped lya sample (DLAs) but consistent with that of sub$-$DLAs. We find the existence of strong anti-correlation between $L$ vs. $[C/H]$ for the combined sample with a significance level of 8.39$sigma$. We see evidence of two distinct $[C/H]$ branch C III populations (low$-[C/H]$ branch, $[C/H]$ $le -1.2$ and high$-[C/H]$ branch, $[C/H]$ $> -1.2$) in the combined C III sample when divided appropriately in the $L$ vs. $N_{rm C,III}$ plane. Further studies of C III absorbers in the redshift range, $1.0 le z le 2.0$ is important to map the redshift evolution of these absorbers and gain insights into the time evolution physical conditions of the circumgalactic medium.
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Using high-resolution UV spectra of 16 low-z QSOs, we study the physical conditions and statistics of O VI absorption in the IGM at z < 0.5. We identify 51 intervening (z_{abs} << z_{QSO}) O VI systems comprised of 77 individual components, and we find 14 proximate systems (z_{abs} ~ z_{QSO}) containing 34 components. For intervening systems [components] with rest-frame equivalent width W_{r} > 30 mA, the number of O VI absorbers per unit redshift dN/dz = 15.6(+2.9/-2.4) [21.0(+3.2/-2.8)], and this decreases to dN/dz = 0.9(+1.0/-0.5) [0.3(+0.7/-0.3)] for W_{r} > 300 mA. The number per redshift increases steeply as z_{abs} approaches z_{QSO}, and some proximate absorbers have substantially lower H I/O VI ratios. The lower proximate ratios could be partially due to ionization effects but also require higher metallicities. We find that 37% of the intervening O VI absorbers have velocity centroids that are well-aligned with corresponding H I absorption. If the O VI and the H I trace the same gas, the relatively small differences in line widths imply the absorbers are cool with T < 10^{5} K. Most of these well-aligned absorbers have the characteristics of metal-enriched photoionized gas. However, the O VI in the apparently simple and cold systems could be associated with a hot phase with T ~ 10^{5.5} K if the metallicity is high enough to cause the associated broad Ly alpha absorption to be too weak to detect. We show that 53% of the intervening O VI systems are complex multiphase absorbers that can accommodate both lower metallicity collisionally-ionized gas with T > 10^{5} K and cold photoionzed gas.
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 study a sample of 17 z>1.5 absorbers selected based on the presence of strong CI absorption lines in SDSS spectra and observed with the ESO-VLT spectrograph X-shooter. We derive metallicities, depletion onto dust, and extinction by dust, and analyse the absorption from MgII, MgI, CaII and NaI that are redshifted into the near infrared wavelength range. We show that most of these CI absorbers have high metallicity and dust content. We detect nine CaII absorptions with $W$(CaII$lambda$3934) >0.23 AA out of 14 systems where we have appropriate wavelength coverage. The observed equivalent widths are similar to what has been measured in other lower redshift surveys of CaII systems. We detect ten NaI absorptions in the 11 systems where we could observe this absorption. The median equivalent width ($W$(NaI$lambda$5891) = 0.68 AA) is larger than what is observed in local clouds with similar HI column densities but also in z<0.7 CaII systems detected in the SDSS. The systematic presence of NaI absorption in these CI systems strongly suggests that the gas is neutral and cold, maybe part of the diffuse molecular gas in the ISM of high-redshift galaxies. Most of the systems (12 out of 17) have $W$(MgII$lambda$2796) > 2.5 AA and six of them have log N(HI) < 20.3, with the extreme case of J1341+1852 that has log N(HI) = 18.18. The MgII absorptions are spread over more than $Delta v$ $sim$ 400 km s$^{-1}$ for half of the systems; three absorbers have $Delta v$ > 500 km s$^{-1}$. The kinematics are strongly perturbed for most of these systems, which probably do not arise in quiet disks and must be close to regions with intense star-formation activity and/or are part of interacting objects. All this suggests that a large fraction of the cold gas at high redshift arises in disturbed environments.
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.
A new method is used to measure the physical conditions of the gas in damped Lyman-alpha systems (DLAs). Using high resolution absorption spectra of a sample of 80 DLAs, we are able to measure the ratio of the upper to lower fine-structure levels of the ground state of C II and Si II. These ratios are determined solely by the physical conditions of the gas. We explore the allowed physical parameter space using a Monte Carlo Markov Chain method to constrain simultaneously the temperature, neutral hydrogen density, and electron density of each DLA. The results indicate that at least 5 % of all DLAs have the bulk of their gas in a dense, cold phase with typical densities of ~100 cm-3 and temperatures below 500 K. We further find that the typical pressure of DLAs in our sample is log(P/k) = 3.4 [K cm-3], which is comparable to the pressure of the local interstellar medium (ISM), and that the components containing the bulk of the neutral gas can be quite small with absorption sizes as small as a few parsec. We show that the majority of the systems are consistent with having densities significantly higher than expected from a purely canonical WNM, indicating that significant quantities of dense gas (i.e. n_H > 0.1 cm-3) are required to match observations. Finally, we identify 8 systems with positive detections of Si II*. These systems have pressures (P/k) in excess of 20000 K cm-3, which suggest that these systems tag a highly turbulent ISM in young, star-forming galaxies.
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