Preprint typeset using LATEX style emulateapj v. 11/10/09 MULTIPHASE GAS IN GALAXY HALOS: THE O VI LYMAN-LIMIT SYSTEM TOWARD J1009+07131

We have serendipitously detected a strong O VI-bearing Lyman limit system at zabs = 0.3558 toward the QSO J1009+0713 (zem = 0.456) in our survey of low-redshift galaxy halos with the Hubble Space Telescope’s Cosmic Origins Spectrograph. Its total rest-frame equivalent width of Wr = 835 ± 49 mÅ and column density of logN(O VI) = 15.0 are the highest for an intervening absorber yet detected in any low-redshift QSO sightline, with absorption spanning at least four major kinematic component groups over 400 km s−1 in its rest frame. HST/WFC3 images of the galaxy field show that the absorber is associated with two galaxies lying at 14 and 46 kpc from the QSO line of sight. The absorber is kinematically complex and there are no less than nine individual Mg IIcomponents spanning 200 km s−1 in our Keck/HIRES optical data. The bulk of the absorbing gas traced by H I resides in two strong, blended component groups that possess a total logN(H I) ' 18 − 18.8, but most of the O VI is associated with two outlying components with logN(H I) = 14.8 and 16.5. The ion ratios an