Mid-infrared spectroscopic evidence for AGN heating warm molecular gas

We analyse 2015 mid-infrared (MIR) spectra of galaxies observed with Spitzer's Infrared Spectrograph, including objects with growing supermassive black holes (SMBHs) and objects where most of the infrared emission originates from newly formed stars. We determine if and how accreting SMBHs at the centre of galaxies-known as active galactic nuclei (AGNs) heat and ionize their host galaxies' dust and molecular gas. We use four MIR diagnostics to estimate the contribution of the AGN to the total MIR emission. We refer to galaxies whose AGNs contribute more than 50 per cent of the total MIR emission as AGN dominated. We compare the relative strengths of polycyclic aromatic hydrocarbon (PAH) emission features and find that PAH grains in AGN-dominated sources have a wider range of sizes and fractional ionizations than PAHgrains in non-AGN-dominated sources. We measure rotational transitions of H-2 and estimate H-2 excitation temperatures and masses for individual targets, H-2 excitation temperatures for spectra stacked by their AGN contribution to the MIR, and the H-2 excitation temperature distributions via a hierarchical Bayesian model. Using the hierarchical Bayesian model, we find an average 200 K difference between the excitation temperatures of the H2S(5) and H2S(7) pure rotational molecular hydrogen transition pair in AGN-dominated versus non-AGN-dominated galaxies. Our findings suggest that AGNs impact the interstellar medium of their host galaxies.