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 Ly alpha 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 Ly alpha absorption to impact parameter rho = 300 h(72)(-1) kpc: 33/37 of our L > 0.01 L* galaxies show Ly alpha equivalent width W-Ly alpha >= 50 m angstrom. 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 similar to 10(10.5 +/- 0.3) M-circle dot for a constant N-H = 10(19) cm(-2). The virialized halos and extended CGM of present-day galaxies are responsible for most strong Ly alpha absorbers (W-Ly alpha > 300 m angstrom) but cannot reproduce the majority of observed lines in the Ly alpha forest. We conclude that the majority of Ly alpha absorption with W-Ly alpha = 30-300 m angstrom occurs in the cosmic web predicted by cosmological simulations and estimate a characteristic width for these filaments of approximate to 400 h(72)(-1) kpc. Regarding O VI, we observe a near unity covering fraction to rho = 200 h(72)(-1) kpc for L > 0.1 L* galaxies and to rho = 300 h(72)(-1) kpc for sub-L* (0.1 L* < L < L*) galaxies. Similar to our Ly alpha results, stronger O VI systems (W-1031 > 70 m angstrom) arise in the virialized halos of L > 0.1 L* galaxies. Unlike Ly alpha, the weaker O VI systems (W-1031 approximate to 30 m angstrom) 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 approximate to 0.3 L* and rarely originates in the so-called warm-hot IGM (predicted by cosmological simulations.