The physical nature of circumgalactic medium absorbers in Simba

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∗ > 1010 M⊙) 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∗ < 1011 M⊙, 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 ∼ 80-95per 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 ∼ 3per cent of overall H i absorption but ∼ 30per cent of Mg ii absorption, with the fraction from satellites decreasing with increasing ion excitation energy.