The Ionization State of Sodium in Galactic Winds

Roughly 80% of ultraluminous infrared galaxies (ULIRGs) show blueshifted absorption in the resonance lines of neutral sodium, indicating that cool winds are common in such objects, as shown by Rupke et al. and by Martin. The neutral sodium (Na I) columns indicated by these absorption lines are ~1013-(3 × 1014) cm-2, a surprising result given that the bolometric luminosities range over a factor of only 4. We show that the gas in ULIRG outflows is likely to be in photoionization equilibrium. The very small ULIRG sample of Goldader et al. demonstrates that the ratio of UV flux to FIR flux varies by a factor ~100 from object to object. While the Goldader et al. sample does not overlap with those of Rupke et al. and Martin, we show that such a large variation in UV flux will produce a similar variation in the column of neutral sodium for a fixed mass flux and density. However, if the cold gas is in pressure equilibrium with a hot outflow with a mass-loss rate similar to the star formation rate, the range of ionization state is significantly smaller. Measurements of the UV flux for objects in the Martin and Rupke et al. catalogs will definitively determine whether photoionization effects are responsible for the wide variation seen in the sodium columns. If they are, a determination of the gas density and mass-loss rate in the cool winds will follow, with attendant improvements in our understanding of wind driving mechanisms and of the feedback effects of galaxies on their surroundings.