Warm molecular hydrogen in outflows from Ultraluminous Infrared Galaxies
  Ultraluminous infrared galaxies (ULIRGs) show on average three times more
emission in the rotational transitions of molecular hydrogen than expected
based on their star formation rates. Using Spitzer archival data we investigate
the origin of excess warm H_2 emission in 115 ULIRGs of the IRAS 1 Jy sample.
We find a strong correlation between H_2 and [FeII] line luminosities,
suggesting that excess H_2 is produced in shocks propagating within neutral or
partially ionized medium. This view is supported by the correlations between
H_2 and optical line ratios diagnostic of such shocks. The galaxies powered by
star formation and those powered by active nuclei follow the same relationship
between H_2 and [FeII], with emission line width being the major difference
between these classes (about 500 and 1000 km/sec, respectively). We conclude
that excess H_2 emission is produced as the supernovae and active nuclei drive
outflows into the neutral interstellar medium of the ULIRGs. A weak positive
correlation between H_2 and the length of the tidal tails indicates that these
outflows are more likely to be encountered in more advanced mergers, but there
is no evidence for excess H_2 produced as a result of the collision shocks
during the final coalescence.
