Probing the Intergalactic Medium/Galaxy Connection. V. On the Origin of Lyα and O VI Absorption at z < 0.2

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 _em = 0.06-0.57. Confirming previous studies, we observe a high covering fraction for Lyα absorption to impact parameter ρ = 300 h ^-1 ₇₂ kpc: 33/37 of our L > 0.01 L* galaxies show Lyα equivalent width W ^Lyα >= 50 mÅ. Galaxies of all luminosity L > 0.01 L* and spectral type are surrounded by a diffuse and ionized circumgalactic medium (CGM), whose baryonic mass is estimated at ~10^{10.5 ± 0.3} M _sun for a constant N _H = 10¹⁹ cm^-2. The virialized halos and extended CGM of present-day galaxies are responsible for most strong Lyα absorbers (W ^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 ^Lyα = 30-300 mÅ occurs in the cosmic web predicted by cosmological simulations and estimate a characteristic width for these filaments of ≈400 h ^-1 ₇₂ kpc. Regarding O VI, we observe a near unity covering fraction to ρ = 200 h ^-1 ₇₂ kpc for L > 0.1 L* galaxies and to ρ = 300 h ^-1 ₇₂ kpc for sub-L* (0.1 L* < L < L*) galaxies. Similar to our Lyα results, stronger O VI systems (W ¹⁰³¹ > 70 mÅ) arise in the virialized halos of L > 0.1 L* galaxies. Unlike Lyα, the weaker O VI systems (W 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 ≈ 0.3 L* and rarely originates in the so-called warm-hot IGM (predicted by cosmological simulations.