THE GAS–GALAXY CONNECTION AT<i>z</i>_abs= 0.35: O VI AND H I ABSORPTION TOWARD J 0943+0531

Abstract

We present observations of HI and OVI absorption systems proximate to a\ngalaxy at z_gal = 0.3529. The absorption was detected serendipitously in Cosmic\nOrigins Spectrograph observations of the low-z QSO J0943+0531 (z_qso = 0.564).\nThe data show two separate clouds along the sightline at an impact parameter of\n95kpc from the galaxy. The first is likely low-metallicity gas falling onto the\ngalaxy. This assessment is based on the high velocity offset of the cloud from\nthe galaxy (delta_v = 365kms) and the weak metal line absorption, combined with\nphotoionization modeling. The second cloud, with only a modest velocity\nseparation from the galaxy (delta_v = 85kms), exhibits very strong OVI\nabsorption qualitatively similar to OVI absorption seen in the Milky Way halo.\nCollisional ionization equilibrium models are ruled out by the metal line\ncolumn density ratios. Photoionization modeling implies a length-scale for the\nOVI cloud of ~0.1-1.2Mpc, which indicates the absorbing gas most likely resides\nwithin the local filamentary structure. This system emphasizes that kinematic\nassociation alone is not sufficient to establish a physical connection to\ngalaxies, even at small impact parameters and velocity separations.\nObservations such as these, connecting galaxies with their gaseous\nenvironments, are becoming increasingly important for understanding galaxy\nevolution and provide constraints for cosmological simulations.\n