Probing the Igm-galaxy Connection at Z < 0.5 I : a Galaxy Survey in Qso Fields and a Galaxy-absorber Cross-correlation Study

We present an imaging and spectroscopic survey of galaxies in fields around QSOs HE 0226−4110, PKS 0405−123, and PG 1216+069. The fields are selected to have ultraviolet echelle spectra available, which uncover 195 Lyα absorbers and 13 OVI absorbers along the three sightlines. We obtain robust redshifts for 1104 galaxies of rest-frame absolute magnitude MR −5log h . −16 and at projected physical distances ρ. 4 h−1 Mpc from the QSOs. HST/WFPC2 images of the fields around PKS 0405−123 and PG 1216+069 are available for studying the optical morphologies of absorbing galaxies. Combining the absorber and galaxy data, we perform a cross-correlation study to understand the physical origin of Lyα and OVI absorbers and to constrain the properties of extended gas around galaxies. The results of our study are (1) both strong Lyα absorbers of log N(HI) ≥ 14 and OVI absorbers exhibit a comparable clustering amplitude as emission-line dominated galaxies and a factor of ≈ 6 weaker amplitude than absorption-line dominated galaxies on co-moving projected distance scales of rp < 3 h−1 Mpc; (2) weak Lyα absorbers of log N(HI) < 13.5 appear to cluster very weakly around galaxies; (3) none of the absorption-line dominated galaxies at rp ≤ 250 h−1 kpc has a corresponding OVI absorber to a sensitive upper limit of W (1031) . 0.03 Å, while the covering fraction of OVI absorbing gas around emission-line dominated galaxies is found to be κ≈ 64%; and (4) high-resolution images of five OVI absorbing galaxies show that these galaxies exhibit disk-like morphologies with mildly disturbed features on the edge. Together, the data indicate that OVI absorbers arise preferentially in gas-rich galaxies. In addition, tidal debris in groups/galaxy pairs may be principally responsible for the observed OVI absorbers, particularly those of W (1031) > 70 mÅ. Subject headings: cosmology: observations—intergalactic medium—quasars: absorption lines