Mining circumgalactic baryons in the low-redshift universe

This paper presents an absorption-line study of the multiphase circumgalactic medium (CGM) based on observations of Lyα, C ii, C iv, Si ii, Si iii, and Si iv absorption transitions in the vicinities of 195 galaxies at redshift z Rh leads to a strict upper limit for the covering fraction of heavy elements of ≈3 per cent (at a 95 per cent confidence level) over d = (1 − 9) Rh. At d < Rh, differential covering fraction between low- and high-ionization gas is observed, suggesting that the CGM becomes progressively more ionized from d < 0.3 Rh to larger distances. Comparing CGM absorption observations at low and high redshifts shows that at a fixed fraction of Rh the CGM exhibits stronger mean absorption at z = 2.2 than at z ∼ 0, and that the distinction is most pronounced in low-ionization species traced by C ii and Si ii absorption lines. We discuss possible pseudo-evolution of the CGM as a result of misrepresentation of halo radius, and present a brief discussion on the implications of these findings.