The Direct Detection of Cool, Metal-enriched Gas Accretion onto Galaxies at z ~ 0.5

Abstract

We report on the discovery of cool gas inflow toward six star-forming galaxies with redshifts z ~ 0.35-1. Analysis of Mg II and Fe II resonance-line absorption in Keck/LRIS spectroscopy of the galaxies reveals positive velocity shifts for cool gas of 80-200 km s^-1 with respect to the host galaxy velocity centroids, and equivalent widths for this inflow of >~ 0.6 Å in five of the six objects. The host galaxies exhibit a wide range of star formation rates (SFRs ~1-40 M _⊙ yr^-1) and have stellar masses similar to that of the Milky Way (log M _*/M _⊙ ~ 9.6-10.5). Imaging from the Hubble Space Telescope Advanced Camera for Surveys indicates that five of the six galaxies have highly inclined (i > 55°), disk-like morphologies. These data represent the first unambiguous detection of inflow into isolated, star-forming galaxies in the distant universe. We suggest that the inflow is due to the infall of enriched material from dwarf satellites and/or a galactic fountain within the galaxies. Assuming that the material has been enriched to 0.1 Z _⊙ and has a physical extent approximately equal to that of the galaxies (implied by the high observed gas covering fractions), we infer mass inflow rates of dM _in/dt >~ 0.2-3 M _⊙ yr^-1 for four of these systems. Finally, from comparison of these absorption lines to the profiles of Mg II and Fe II absorption in a larger spectroscopic sample of ~100 objects, we measure a covering fraction of cool inflow of at least 6%, but cannot rule out the presence of enriched infall onto as many as ~40 of these galaxies.