Warm Molecular Hydrogen in Nearby, Luminous Infrared Galaxies

Abstract

Mid-infrared molecular hydrogen (H_2) emission is a powerful cooling agent in galaxy mergers and in radio galaxies; it is a potential key tracer of gas evolution and energy dissipation associated with mergers, star formation, and accretion onto supermassive black holes. We detect mid-IR H_2 line emission in at least one rotational transition in 91% of the 214 Luminous Infrared Galaxies (LIRGs) observed with Spitzer as part of the Great Observatories All-sky LIRG Survey. We use H_2 excitation diagrams to estimate the range of masses and temperatures of warm molecular gas in these galaxies. We find that LIRGs in which the IR emission originates mostly from the Active Galactic Nuclei (AGN) have about 100 K higher H_2 mass-averaged excitation temperatures than LIRGs in which the IR emission originates mostly from star formation. Between 10% and 15% of LIRGs have H_2 emission lines that are sufficiently broad to be resolved or partially resolved by the high-resolution modules of Spitzer's Infrared Spectrograph (IRS). Those sources tend to be mergers and contain AGN. This suggests that a significant fraction of the H_2 line emission is powered by AGN activity through X-rays, cosmic rays, and turbulence. We find a statistically significant correlation between the kinetic energy in the H2 gas and the H_2 to IR luminosity ratio. The sources with the largest warm gas kinetic energies are mergers. We speculate that mergers increase the production of bulk inflows leading to observable broad H_2 profiles and possibly denser gas.

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