Mining circumgalactic baryons in the low-redshift universe

Abstract

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 < 0.176. The galaxy sample is established based on a cross-comparison between public galaxy and quasi-stellar object (QSO) survey data and is characterized by a median redshift of < z > = 0.041, a median projected distance of < d > = 362 kpc to the sightline of the background QSO, and a median stellar mass of log (M_star/M_⊙) = 9.7 ± 1.1. Comparing the absorber features identified in the QSO spectra with known galaxy properties has led to strong constraints for the CGM absorption properties at z ≲ 0.176. First, abundant hydrogen gas is observed out to d ∼ 500 kpc, well beyond the dark matter halo radius R_h of individual galaxies, with a mean covering fraction of ≈60 per cent. In contrast, no heavy elements are detected at d ≳ 0.7 R_h from either low-mass dwarfs or high-mass galaxies. The lack of detected heavy elements in low- and high-ionization states suggests that either there exists a chemical enrichment edge at d ≈ 0.7 R_h or gaseous clumps giving rise to the observed absorption lines cannot survive at these large distances. Considering all galaxies at d > R_h 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) R_h. At d < R_h, differential covering fraction between low- and high-ionization gas is observed, suggesting that the CGM becomes progressively more ionized from d < 0.3 R_h to larger distances. Comparing CGM absorption observations at low and high redshifts shows that at a fixed fraction of R_h 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.