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 (IV). [11.7, 14.1], N-C III proportional to 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 CIV components tied (aligned) with H I at log N-H I is an element of [12.2, 16.0] and log N-C IV. [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 similar to 4.4 +/- 0.3 for log T-b is an element of [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 is an element of [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) is an element of [-4.5, -3.3] and log T-b is an element of [3.9, 4.5]. Low-metallicity branch absorbers have [C/H](temp) <= -1.0, L-temp is an element of [20, 480] kpc and log n(H,temp) is an element of [-5.2, -4.3] and log T-b similar to 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.