Evidence of bimodal physical properties of intervening, optically thin C III absorbers at z ∼ 2.5

We present the Voigt profile analysis of 132 intervening C IV+C III components associated with optically thin H I absorbers at 2.1 < z < 3.4 in the 19 high-quality UVES/VLT and HIRES/Keck QSO spectra. For log N_{C I} in [11.7, 14.1], N_{C III} ∝ N_{C IV}^{1.42± 0.11} and < N_{C III}/N_{C {IV} > = 1.0± 0.3 with a negligible redshift evolution. For 54 C IV components tied (aligned) with H I at log N_{H I} in [12.2, 16.0] and log N_{C IV} in [11.8, 13.8], the gas temperature T_b estimated from absorption line widths is well approximated to a Gaussian peaking at log T_b ∼ 4.4 ± 0.3 for log T_b ∈ [3.5, 5.5], with a negligible non-thermal contribution. For 32 of 54 tied H I+C IV pairs, also tied with C III at log N_{C {III} in [11.7, 13.8], we ran both photoionization equilibrium (PIE) and non-PIE (using a fixed temperature T_b) CLOUDY models for the Haardt-Madau QSOs+galaxies 2012 UV background. We find evidence of bimodality in observed and derived physical properties. High-metallicity branch absorbers have a carbon abundance [C/H]_temp ≥ -1.0, a line-of-sight length L_temp ≤ 20 kpc and a total (neutral and ionized) hydrogen volume density log n_H,temp ∈ [-4.5, -3.3] and log T_b ∈ [3.9, 4.5]. Low-metallicity branch absorbers have [C/H]_temp ≤ -1.0, L_temp ∈ [20, 480] kpc and log n_H,temp ∈ [-5.2, -4.3] and log T_b ∼ 4.5. High-metallicity branch absorbers seem to be originated from extended discs, inner haloes or outflowing gas of intervening galaxies, while low-metallicity absorbers are produced by galactic haloes or the surrounding intergalactic medium filament.