The gas-galaxy connection at zabs = 0.35: O VI and HI absorption toward J 0943+0531

Abstract

We present observations of HI and O VI absorption systems proximate to a galaxy at zgal = 0.3529. The absorption was detected serendipitously in Cosmic Origins Spectrograph observations of the low-z QSO J 0943 + 0531 (zqso = 0.564). The data show two separate clouds along the sightline at an impact parameter of 95 kpc 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 (Δv = 365 km s -1) and the weak metal line absorption, combined with photoionization modeling. The second cloud, with only a modest velocity separation from the galaxy (Δv = 85 km s-1), exhibits very strong O VI absorption qualitatively similar to O VI 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 O VI cloud of ∼0.1-1.2 Mpc, 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 of galaxy evolution and provide constraints for cosmological simulations. © 2011. The American Astronomical Society. All rights reserved.