The physical nature of circumgalactic medium absorbers in Simba

Abstract

We study the nature of the low-redshift circumgalactic medium (CGM) in the SIMBA  cosmological simulations as traced by ultraviolet absorption lines around galaxies in bins of stellar mass (⁠M⋆>1010M⊙⁠) for star-forming, green valley and quenched galaxies at impact parameters r⊥ ≤ 1.25r200. We generate synthetic spectra for H I , Mg II , C II , Si III , C IV , and O VI , fit Voigt profiles to obtain line properties, and estimate the density, temperature, and metallicity of the absorbing gas. We find that CGM absorbers are most abundant around star-forming galaxies with M⋆<1011M⊙⁠, while the abundance of green valley galaxies show similar behaviour to those of quenched galaxies, suggesting that the CGM ‘quenches’ before star formation ceases. H I  absorbing gas exists across a broad range of cosmic phases [condensed gas, diffuse gas, hot halo gas, and Warm-Hot Intergalactic Medium (WHIM)], while essentially all low ionization metal absorption arises from condensed gas. O VI absorbers are split between hot halo gas and the WHIM. The fraction of collisionally ionized CGM absorbers is ∼25--55 per cent for C IV  and ∼80--95 per cent for O VI , depending on stellar mass and impact parameter. In general, the highest column density absorption features for each ion arise from dense gas. Satellite gas, defined as that within 10r1/2,⋆, contributes ∼3 per cent of overall H I absorption but ∼30 per cent of Mg II  absorption, with the fraction from satellites decreasing with increasing ion excitation energy.