Evidence of bimodal physical properties of intervening, optically-thin CIII absorbers at z ~ 2.5

TL;DR

This study analyzes intervening CIII absorbers at z ~ 2.5, revealing bimodal physical properties linked to different galactic environments, using Voigt profile analysis and photoionization modeling.

Contribution

It provides the first evidence of bimodality in the physical properties of CIII absorbers at high redshift, distinguishing between high- and low-metallicity populations.

Findings

High-metallicity absorbers have shorter line-of-sight lengths and higher densities.

Low-metallicity absorbers are more extended and less dense.

Bimodal properties suggest different origins, such as galaxy disks and IGM filaments.

Abstract

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.