MULTIPHASE GAS IN GALAXY HALOS: THE O VI LYMAN-LIMIT SYSTEM TOWARD J1009+0713

Abstract

We have serendipitously detected a strong O VI-bearing Lyman limit system at\nz_abs = 0.3558 toward the QSO J1009+0713 (z_em = 0.456) in our survey of\nlow-redshift galaxy halos with the Hubble Space Telescope's Cosmic Origins\nSpectrograph. Its rest-frame equivalent width of W_r = 835 +/- 49 mA is the\nhighest for an intervening absorber yet detected in any low-redshift QSO\nsightline, with absorption spanning 400 km s^-1 in its rest frame. HST/WFC3\nimages of the galaxy field show that the absorber is associated with two\ngalaxies lying at 14 and 46 kpc from the QSO line of sight. The bulk of the\nabsorbing gas traced by H I resides in two strong, blended component groups\nthat possess a total logN(HI) = 18 - 18.8. The ion ratios and column densities\nof C, N, O, Mg, Si, S, and Fe, except the O VI, can be accommodated into a\nsimple photoionization model in which diffuse, low-metallicity halo gas is\nexposed to a photoionizing field from stars in the nearby galaxies that\npropagates into the halo at 10% efficiency. We constrain the metallicity firmly\nwithin the range 0.1 - 1 Zsun, and photoionization modeling indirectly\nindicates a subsolar metallicity of 0.05 - 0.5 Zsun. The appearance of strong O\nVI and nine Mg II components and our review of similar systems in the\nliterature support the "interface" picture of high-velocity O VI: the total\nstrength of the O VI shows a positive correlation with the number of detected\ncomponents in the low-ionization gas, however the total O VI column densities\nstill far exceed the values expected from interface models for the number of\ndetected clouds.\n