Lyα ESCAPE FROM z ∼ 0.03 STAR-FORMING GALAXIES: THE DOMINANT ROLE OF OUTFLOWS

The usefulness of H I Ly alpha photons for characterizing star formation in the distant universe is limited by our understanding of the astrophysical processes that regulate their escape from galaxies. These processes can only be observed in detail out to a few x 100 Mpc. Past nearby (z < 0.3) spectroscopic studies are based on small samples and/or kinematically unresolved data. Taking advantage of the high sensitivity of Hubble Space Telescope's Cosmic Origins Spectrograph (COS), we observed the Ly alpha lines of 20 H alpha-selected galaxies located at < z > = 0.03. The galaxies cover a broad range of luminosity, oxygen abundance, and reddening. In this paper, we characterize the observed Ly alpha lines and establish correlations with fundamental galaxy properties. We find seven emitters. These host young (<= 10 Myr) stellar populations have rest-frame equivalent widths in the range 1-12 angstrom, and have Ly alpha escape fractions within the COS aperture in the range 1%-12%. One emitter has a double-peaked Ly alpha with peaks 370 km s(-1) apart and a stronger blue peak. Excluding this object, the emitters have Ly alpha and O I lambda 1302 offsets from II alpha in agreement with expanding-shell models and Lyman break galaxies observations. The absorbers have offsets that are almost consistent with a static medium. We find no one-to-one correspondence between Ly alpha emission and age, metallicity, or reddening. Thus, we confirm that Ly alpha is enhanced by outflows and is regulated by the dust and H I column density surrounding the hot stars.