Jet-Shocked H2 and CO in the Anomalous Arms of Molecular Hydrogen Emission Galaxy NGC 4258

TL;DR

This study uses infrared spectroscopy to analyze jet-induced shocks in molecular hydrogen within NGC 4258, revealing heated gas, altered CO-to-H2 ratios, and evidence of jet-driven gas ejection into the galaxy halo.

Contribution

It provides detailed spatial mapping of shocked H2 and CO emissions, and proposes a lower X(CO) conversion factor due to high temperature and turbulence, advancing understanding of jet-gas interactions.

Findings

H2 emission originates from 9.4 million solar masses of warm molecular gas.

The X(CO) factor is estimated to be ten times lower than the Milky Way value.

Significant gas ejection into the galaxy halo due to jet interaction.

Abstract

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.