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 infraredgalaxies (ULIGs) have been combined with published data, in order to reassessthe dependence of galactic outflow speeds on starburst luminosity and galacticmass. The Doppler shifts reveal outflows of relatively cool gas at $330 \pm100$km/s. The outflow speeds increase with the star formation rate (SFR) asroughly $v \propto SFR^{0.35}$. This result is surprising since, in thetraditional model for starburst-driven winds, these relatively cool gas cloudsare accelerated by the ram pressure of a hot, supernova-heated wind thatexhibits weak (if any) \x temperature variation with increasing galactic mass.The lack of evidence for much hotter winds is partly a sensitivity issue; butthe outflow velocities in ultraluminous starbursts actually are consistent withacceleration by the tepid wind, indicating a hotter component is unlikely todominate the momentum flux. The \nad velocities in the dwarf starburst winds donot 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 starburstwinds simply lack enough momentum to accelerate the clouds to the velocity ofthe hot wind. Among the subsample of starbursts with well- constraineddynamical masses, the terminal outflow velocities approach the galactic escapevelocity. If this relation, which is similar to that observed for stellarwinds, arises from radiation pressure on dust and gas limiting the maximumluminosity of a starburst, then feedback will be stronger in massive galaxiesthan previously thought helping shape the high-mass end of the galaxyluminosity function.Comment: 12 pages+figures and tables; submitted to Ap