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Mapping Large-Scale Gaseous Outflows in Ultraluminous Galaxies with Keck II ESI Spectra: Variations in Outflow Velocity with Galactic Mass

Measurements of interstellar absorption lines in 18 ultraluminous infrared galaxies (ULIGs) have been combined with published data, in order to reassess the dependence of galactic outflow speeds on starburst luminosity and galactic mass. The Doppler shifts reveal outflows of relatively cool gas at $330 \pm 100$km/s. The outflow speeds increase with the star formation rate (SFR) as roughly $v \propto SFR^{0.35}$. This result is surprising since, in the traditional model for starburst-driven winds, these relatively cool gas clouds are accelerated by the ram pressure of a hot, supernova-heated wind that exhibits weak (if any) \x temperature variation with increasing galactic mass. The lack of evidence for much hotter winds is partly a sensitivity issue; but the outflow velocities in ultraluminous starbursts actually are consistent with acceleration by the tepid wind, indicating a hotter component is unlikely to dominate the momentum flux. The \nad velocities in the dwarf starburst winds do not reach the terminal velocity of a hot wind at the measured temperature of kT ~0.73 keV. A dynamically-motivated explanation is that the dwarf starburst winds simply lack enough momentum to accelerate the clouds to the velocity of the hot wind. Among the subsample of starbursts with well- constrained dynamical masses, the terminal outflow velocities approach the galactic escape velocity. If this relation, which is similar to that observed for stellar winds, arises from radiation pressure on dust and gas limiting the maximum luminosity of a starburst, then feedback will be stronger in massive galaxies than previously thought helping shape the high-mass end of the galaxy luminosity function.