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 ≤ z ≤ 0.9 associated with neutral hydrogen column densities in the range 15 ≤ log $N({\rm{H\,{\small I}}})$ (cm^-2) ≤ 16.2. Using photoionization models, we infer the number density (n_H), C-abundance ([C/H]), and line-of-sight thickness (L) of these systems in the ranges -3.4 ≤ log n_H (cm^-3) ≤ -1.6, -1.6 ≤ [C/H] ≤ 0.4, and 1.3 pc ≤ L ≤ 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 ≤ z ≤ 3.3) optically thin C III systems to study the redshift evolution and various correlations between the derived physical parameters. We see a significant redshift evolution in n_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] versus z for C III absorbers is steeper compared to the redshift evolution of cosmic metallicity of the damped Ly α sample (DLA) but consistent with that of sub-DLAs. We find the existence of strong anticorrelation between L and [C/H] for the combined sample with a significance level of 8.39σ. We see evidence of two distinct [C/H] branch C III populations (low-[C/H] branch, $[\mathrm{ C/H}]\, \le -1.2$ , and high-[C/H] branch, $[\mathrm{ C/H}]\, \gt -1.2$ ) in the combined C III sample when divided appropriately in the L versus $N({\rm{C\,{\small III}}})$ plane. Further studies of C III absorbers in the redshift range 1.0 ≤ z ≤ 2.0 are important to map the redshift evolution of these absorbers and gain insights into the time evolution physical conditions of the circumgalactic medium.