2014ApJ...788L..33O

We present a Spitzer Infrared Spectrograph map of H₂ emission from the nearby galaxy NGC 4258 (Messier 106). The H₂emission comes from 9.4±0.4x10⁶ M_☉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 H₂over 7.7 µm polycyclic aromatic hydrocarbon emission (0.37), characteristic of shocked molecular gas. We find close spatial correspondence between the H₂and 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 1.0x10⁸ M_☉based on dust emission. We suggest that the X_COvalue is 10 times lower than the Milky Way value because of high kinetic temperature and enhanced turbulence. The H₂disk 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 M_☉/yr in the central 3.4 kpc² that is consistent with the remaining gas surface density.