Physical conditions and redshift evolution of optically thin C III absorbers: Low-z sample

Abstract

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, $15\le \log$ $N_{\mathrm{H}\mathrm{I}}$ ($c{m}^{-2}$) $\le 16.2$. Using photoionization models, we infer the number density ($n_{\mathrm{H}}$), C-abundance ($[C/H]$) and line-of-sight thickness ($L$) of these systems in the ranges, $-3.4\le$ log $n_{\mathrm{H}}$ (in $c{m}^{-3}$) $\le -1.6$, $-1.6\le [C/H]\le 0.4$, 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$-z$ ($2.1\le z\le 3.3$) 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 $n_{\mathrm{H}}$, $[C/H]$ 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. $N_{\mathrm{C}\mathrm{III}}$ 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.

arXiv commentAccepted for publishing in MNRAS, comments are welcome

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