Evidence of bimodal physical properties of intervening, optically-thin
  CIII absorbers at z ~ 2.5
  We present the Voigt profile analysis of 132 intervening CIV+CIII components
associated with optically-thin HI absorbers at 2.1 < z < 3.4 in the 19
high-quality UVES/VLT and HIRES/Keck QSO spectra. For log N(CIV) = [11.7,
14.1], N(CIII) is proportional to N(CIV) with an exponent (1.42 +- 0.11) and <
N(CIII)/N(CIV) > = 1.0 +- 0.3 with a negligible redshift evolution. For 54 CIV
components tied (aligned) with HI at log N(HI) = [12.2, 16.0] and log N(CIV) =
[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
HI+CIV pairs, also tied with CIII at log N(CIII) = [11.7, 13.8], we ran both
photoionisation equilibrium (PIE) and non-PIE (using a fixed temperature T_b)
Cloudy models for the Haardt-Madau QSO+galaxy 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 ionised)
hydrogen volume density log n(H, temp) = [-4.5, -3.3] and 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 disks, inner halos or
outflowing gas of intervening galaxies, while low-metallicity absorbers are
produced by galactic halos or the surrounding IGM filament.
