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, $15\le \mathrm{l}\mathrm{o}\mathrm{g}$ $N_{\mathrm{H}\mathrm{I}}$ ($c{m}^{-2}$) $\le 16.2$\mathrm{.}Usingphotoionizationmodels,weinferthenumberdensity($n_{\rm H}$), C-abundance ($[C\mathrm{/}H]$) and line-of-sight thickness ($L$) of these systems in the ranges, $-3.4\le$ log $n_{\mathrm{H}}$ (in $c{m}^{-3}$) $\le -1.6$, $-1.6 \le [C/H] \le 0.4$,and1.3pc$\le L \le$ 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 $n_{\mathrm{H}}$, $[C\mathrm{/}H]$ 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 $[C\mathrm{/}H]$ 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. $[C\mathrm{/}H]$ for the combined sample with a significance level of 8.39$\sigma$. We see evidence of two distinct $[C\mathrm{/}H]$ branch C III populations (low$-[C\mathrm{/}H]$ branch, $[C\mathrm{/}H]$ $\le -1.2$ and high$-[C\mathrm{/}H]$ branch, $[C\mathrm{/}H]$ $>-1.2$) in the combined C III sample when divided appropriately in the $L$ vs. $N_{\rm C,III}$plane\mathrm{.}FurtherstudiesofCIIIabsorbersintheredshiftrange,$1.0 \le z \le 2.0$ is important to map the redshift evolution of these absorbers and gain insights into the time evolution physical conditions of the circumgalactic medium.