Probing the IGM-galaxy connection at z < 0.5-II. New insights into the galaxy environments of O VI absorbers in PKS 0405-123

We present new absorption-line analysis and new galaxy survey data obtained for the field around PKS 0405-123 at z(QSO) = 0.57. Combining previously known O vi absorbers with new identifications in the higher S/N ultraviolet (UV) spectra obtained with the Cosmic Origins Spectrograph, we have established a sample of 7 O vi absorbers and 12 individual components at z = 0.0918-0.495 along the sightline towards PKS 0405-123. We complement the available UV absorption spectra with galaxy survey data that reach 100 per cent completeness at projected distances < 200 kpc of the quasar sightline for galaxies as faint as 0.1 L-* (0.2 L-*) out to redshifts of z approximate to 0.35 (z approximate to 0.5). The high level of completeness achieved at faint magnitudes by our survey reveals that O vi absorbers are closely associated with gas-rich environments containing at least one low-mass, emission-line galaxy. An intriguing exception is a strong O vi system at z approximate to 0.183 that does not have a galaxy found at < 4 Mpc, and our survey rules out the presence of any galaxies of L > 0.04 L-* at < 250 kpc and any galaxies of L > 0.3 L-* at < 1 Mpc. We further examine the galactic environments of O vi absorbers and those 'Ly alpha-only' absorbers with neutral hydrogen column density log N(Hi < 13.6 and no detectable O vi absorption features. The Ly alpha-only absorbers serve as a control sample in seeking the discriminating galactic features that result in the observed O vi absorbing gas at large galactic radii. We find a clear distinction in the radial profiles of mean galaxy surface brightness around different absorbers. Specifically, O vi absorbers are found to reside in regions of higher mean surface brightness at less than or similar to 500 kpc (delta mu(R) approximate to +5 mag Mpc(-2) relative to the background at > 500 kpc), while only a mild increase in galaxy surface brightness is seen at small around Ly alpha-only absorbers (delta mu(R) approximate to +2 mag Mpc(-2)). The additional insights gained from our deep galaxy survey demonstrate the need to probe the galaxy populations to low luminosities in order to better understand the nature of the absorbing systems.