Jet-Shocked H2 and CO in the Anomalous Arms of Molecular Hydrogen
  Emission Galaxy NGC 4258
  We present a Spitzer Infrared Spectrograph (IRS) map of H2 emission from the
nearby galaxy NGC 4258 (Messier 106). The H2 emission comes from 9.4E6 Msun of
warm molecular hydrogen heated to 240-1040 K in the inner anomalous arms, a
signature of jet interaction with the galaxy disk. The spectrum is that of a
molecular hydrogen emission galaxy (MOHEG), with a large ratio of H2 over 7.7
micron PAH emission (0.37), characteristic of shocked molecular gas. We find
close spatial correspondence between the H2 and CO emission from the anomalous
arms. Our estimate of cold molecular gas mass based on CO emission is 10 times
greater than our estimate of 1.0E8 Msun based on dust emission. We suggest that
the X(CO) value is 10 times lower than the Milky Way value because of high
kinetic temperature and enhanced turbulence. The H2 disk has been overrun and
is being shocked by the jet cocoon, and much of the gas originally in the disk
has been ejected into the galaxy halo in an X-ray-hot outflow. We measure a
modest star formation rate of 0.08 Msun/yr in the central 3.4 square kpc that
is consistent with the remaining gas surface density.
