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

We present new absorption-line analysis and new galaxy survey data obtained for the field around PKS0405-123 at z=0.57. Combining previously known OVI absorbers with new identifications in the higher S/N UV spectra obtained with COS, we have established a sample of 7 OVI absorbers and 12 individual components at z=0.0918-0.495 along the sightline toward PKS0405-123. We complement the available UV absorption spectra with galaxy survey data that reach 100% completeness at projected distances rho<200 kpc of the quasar sightline for galaxies as faint as 0.1L* (0.2L*) out to redshifts of z=0.35 (z=0.5). The high level of completeness achieved at faint magnitudes by our survey reveals that OVI absorbers are closely associated with gas-rich environments containing at least one low-mass, emission-line galaxy. An intriguing exception is a strong OVI system at z=0.183 that does not have a galaxy found at rho<4 Mpc, and our survey rules out the presence of any galaxies of L>0.04L* at rho<250 kpc and any galaxies of L>0.3L* at rho<1 Mpc. We further examine the galactic environments of OVI absorbers and those "Lya-only" absorbers with HI column density log N(HI)>13.6 and no detectable OVI absorption features. The Lya-only absorbers serve as a control sample in seeking the discriminating galactic features that result in the observed OVI absorbing gas at large galactic radii. We find a clear distinction in the radial profiles of mean galaxy surface brightness around different absorbers. Specifically, OVI absorbers are found to reside in regions of higher mean surface brightness at rho<500 kpc (+5 mag Mpc^-2 relative to the background), while only a mild increase in galaxy surface brightness is seen at small rho around Lya-only absorbers (+2 mag Mpc^-2). The additional insights gained from our deep galaxy survey demonstrates the need to probe to low luminosities to better understand absorbing systems.