The Gas-Galaxy Connection at z = 0.35: OVI and HI Absorption Towards J0943+0531

Abstract

We present observations of HI and OVI absorption systems proximate to a galaxy at z_gal = 0.3529. The absorption was detected serendipitously in Cosmic Origins Spectrograph observations of the low-z QSO J0943+0531 (z_qso = 0.564). The data show two separate clouds along the sightline at an impact parameter of 95kpc from the galaxy. The first is likely low-metallicity gas falling onto the galaxy. This assessment is based on the high velocity offset of the cloud from the galaxy (delta_v = 365kms) and the weak metal line absorption, combined with photoionization modeling. The second cloud, with only a modest velocity separation from the galaxy (delta_v = 85kms), exhibits very strong OVI absorption qualitatively similar to OVI absorption seen in the Milky Way halo. Collisional ionization equilibrium models are ruled out by the metal line column density ratios. Photoionization modeling implies a length-scale for the OVI cloud of ~0.1-1.2Mpc, which indicates the absorbing gas most likely resides within the local filamentary structure. This system emphasizes that kinematic association alone is not sufficient to establish a physical connection to galaxies, even at small impact parameters and velocity separations. Observations such as these, connecting galaxies with their gaseous environments, are becoming increasingly important for understanding galaxy evolution and provide constraints for cosmological simulations.Comment: ApJ in press. 11 pages, 7 figure