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.