PROBING THE INTERGALACTIC MEDIUM/GALAXY CONNECTION. V. ON THE ORIGIN OF Lyα AND O VI ABSORPTION AT z < 0.2

Abstract

We analyze the association of galaxies with Lyα and O VI absorption, the most commonly detected transitions of the low-z intergalactic medium (IGM), in the fields of 14 quasars with z[subscript em] = 0.06–0.57. Confirming previous studies, we observe a high covering fraction for Lyα absorption to impact parameter ρ = 300 h[superscript −1][subscript 72] kpc: 33/37 of our L > 0.01L* galaxies show Lyα equivalent width W[superscript Lyα] 50 [bar under >] 50 mÅ. Galaxies of all luminosity L > 0.01L* and spectral type are surrounded by a diffuse and ionized circumgalactic medium (CGM), whose baryonic mass is estimated at ∼10[superscript 10.5±0.3]M [subscript ☉] for a constant N[subscript H] = 10[superscript 19] cm[superscript −2]. The virialized halos and extended CGM of present-day galaxies are responsible for most strong Lyα absorbers (W[superscript Lyα] > 300 mÅ) but cannot reproduce the majority of observed lines in the Lyα forest. We conclude that the majority of Lyα absorption with W[superscript Lyα] = 30–300 mÅ occurs in the cosmic web predicted by cosmological simulations and estimate a characteristic width for these filaments of ≈400 h[superscript −1][subscript 72] kpc. Regarding O VI, we observe a near unity covering fraction to ρ = 200 h[superscript −1][subscript 72] kpc for L > 0.1L* galaxies and to ρ = 300 h[superscript −1][subscript 72] kpc for sub-L*(0.1L* <L <L*) galaxies. Similar to our Lyα results, stronger OVI systems (W[superscript 1031] > 70mÅ) arise in the virialized halos of L > 0.1L* galaxies. Unlike Lyα, the weaker O VI systems (W[superscript 1031] ≈ 30 mÅ) arise in the extended CGM of sub-L* galaxies. The majority of O [VI] gas observed in the low-z IGM is associated with a diffuse medium surrounding individual galaxies with L ≈ 0.3L* and rarely originates in the so-called warm-hot IGM (predicted by cosmological simulations.