  Roughly 80% of Ultraluminous Infrared Galaxies (ULIRGs) show blue shifted
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 (NaI) columns indicated by these absorption lines are ~
10^{13}-3x10^{14}/cm^2, while the bolometric luminosity varies by a factor of
only four. 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 ultraviolet flux to far infrared flux varies by
a factor $\sim100$ from object to object. While the Goldader sample does not
overlap with those of Rupke et al. and Martin, we show that such a large
variation in ultraviolet 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 if 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 effects
of galaxies on their surroundings.
