The physical nature of circumgalactic medium absorbers in Simba

Abstract

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 ($\mbox{$M_\star $}\, > 10^{10}{\rm M}_\odot$) 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 $\mbox{$M_\star $}\, < 10^{11}\,\,{\rm M}_{\odot }$, 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 $\sim 25{\text{--}}55{{\ \rm per\ cent}}$ for C iv  and $\sim 80{\text{--}}95{{\ \rm per\ cent}}$ 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 $\sim 3{{\ \rm per\ cent}}$ of overall H i  absorption but $\sim 30{{\ \rm per\ cent}}$ of Mg ii  absorption, with the fraction from satellites decreasing with increasing ion excitation energy.