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.

Accepted for publishing in MNRAS, comments are welcome

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