Outflows in Infrared-Luminous Galaxies: Absorption-Line Spectroscopy of Starbursts and AGN
Rupke, David Stuart
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Large-scale galactic outflows, better known as superwinds, are driven by the powerful energy reservoirs in star forming and active galaxies. They play a significant role in galaxy formation, galaxy evolution, and the evolution of the intergalactic medium. We have performed a survey of over 100 infrared-luminous galaxies in order to address the exact frequency with which they occur in different galaxy types, the dependence of their properties on those of their host galaxies, and their properties in the most luminous starburst and active galaxies. Most of our sample consists of ultraluminous infrared galaxies (ULIRGs), and we use moderate-resolution spectroscopy of the NaI D interstellar absorption feature (which directly probes the neutral gas phase). We find superwinds in the majority of these galaxies at typical maximum, de-projected velocities of 500-700 km/s. The detection rate increases with star formation rate (SFR) in starbursts, while the mass outflow rate appears constant with SFR, contrary to theoretical expectations. The resulting mass entrainment efficiencies in ULIRGs are quite low, of order a few percent of the star formation rate. There is some dependence of outflow velocity on host galaxy properties; the outflow velocities in LINERs are higher than those in HII galaxies, and the highest column density gas in each galaxy may have an upper envelope in velocity that increases with SFR. Outflows in most galaxies hosting a dominant AGN have very similar properties to those in starbursts, so discerning their power source is difficult. The velocities in Seyfert 2 outflows may be slightly higher than those in starbursts, and the fraction of neutral gas escaping Seyfert 2s is higher than that in starbursts (~50% vs. <=20%). The outflows in our Seyfert 1 galaxies have extreme velocities of up to ~10^4 km/s, and two of three Seyfert 1s with outflows possess broad absorption lines. Finally, we find that spectroscopy of a few galaxies at very high spectral resolution does not reveal unresolved narrow components. The mass outflow rates at very high resolution are thus comparable to those from our large sample of moderate-resolution spectra, demonstrating the reliability of our moderate-resolution data.