Absorption-Line Probes of the Prevalence and Properties of Outflows in Present-Day Star-Forming Galaxies

We analyze star-forming galaxies drawn from SDSS DR7 to show how the interstellar medium (ISM) Na i λλ5890, 5896 (Na D) absorption lines depend on galaxy physical properties, and to look for evidence of galactic winds. We combine the spectra of galaxies with similar geometry/physical parameters to create composite spectra with signal-to-noise ∼300. The stellar continuum is modeled using stellar population synthesis models, and the continuum-normalized spectrum is fit with two Na i absorption components. We find that (1) ISM Na D absorption lines with equivalent widths EW > 0.8 Å are only prevalent in disk galaxies with specific properties—large extinction ( A V ), high star formation rates (SFR), high SFR per unit area (Σ SFR ), or high stellar mass ( M * ); (2) the ISM Na D absorption lines can be separated into two components: a quiescent disk-like component at the galaxy systemic velocity and an outflow component; (3) the disk-like component is much stronger in the edge-on systems, and the outflow component covers a wide angle but is stronger within 60° of the disk rotation axis; (4) the EW and covering factor of the disk component correlate strongly with dust attenuation, highlighting the importance that dust shielding may play in the survival of Na i ; (5) the EW of the outflow component depends primarily on Σ SFR and secondarily on A V ; and (6) the outflow velocity varies from ∼120 to 160 km s −1 but shows little hint of a correlation with galaxy physical properties over the modest dynamic range that our sample probes (1.2 dex in log Σ SFR and 1 dex in log M * ).