The Ionization State of Sodium in Galactic Winds

Roughly 80% of Ultraluminous Infrared Galaxies (ULIRGs) show blue shiftedabsorption in the resonance lines of neutral sodium, indicating that cool windsare common in such objects, as shown by Rupke et al and by Martin. The neutralsodium (NaI) columns indicated by these absorption lines are ~10^{13}-3x10^{14}/cm^2, while the bolometric luminosity varies by a factor ofonly four. We show that the gas in ULIRG outflows is likely to be inphotoionization equilibrium. The very small ULIRG sample of Goldader et al.demonstrates that the ratio of ultraviolet flux to far infrared flux varies bya factor $\sim100$ from object to object. While the Goldader sample does notoverlap with those of Rupke et al. and Martin, we show that such a largevariation in ultraviolet flux will produce a similar variation in the column ofneutral sodium for a fixed mass flux and density. However, if the cold gas isin pressure equilibrium with a hot outflow with a mass loss rate similar to thestar formation rate, the range of ionization state is significantly smaller.Measurements of the UV flux for objects in the Martin and Rupke et al. catalogswill definitively determine if photoionization effects are responsible for thewide variation seen in the sodium columns. If they are, a determination of thegas density and mass loss rate in the cool winds will follow, with attendantimprovements in our understanding of wind driving mechanisms and of the effectsof galaxies on their surroundings.Comment: 14 pages, 5 figures, submitted to Ap