Jet-Shocked H_2 and CO in the Anomalous Arms of Molecular Hydrogen Emission Galaxy NGC 4258

Abstract

We present a Spitzer Infrared Spectrograph (IRS) map of H+2 emission from the nearby galaxy NGC 4258 (Messier 106). The H_2 emission comes from 9.4 ± 0.4 × 10^6M_⊙ 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 H_2 over 7.7μm PAH emission (0.37), characteristic of shocked molecular gas. We find close spatial correspondence between the H_2 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 1.0×10^8M_⊙ 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 H_2 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 M_⊙ yr^(−1) in the central 3.4 kpc^2 that is consistent with the remaining gas surface density.

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