Physical conditions and redshift evolution of optically thin C III
  absorbers: Low-z sample
  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$-z$ ($2.1\le z\le 3.3$) 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 $n_{\rm H}$, $[C/H]$ 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.
