Mid-Infrared Spectroscopic Evidence for AGN Heating Warm Molecular Gas

TL;DR

This study analyzes mid-infrared spectra of over 2,000 galaxies to investigate how active galactic nuclei influence the heating and ionization of molecular gas and dust within their host galaxies.

Contribution

It provides new insights into the impact of AGN on the interstellar medium by using MIR diagnostics, PAH feature analysis, and H2 excitation temperature measurements across a large galaxy sample.

Findings

AGN-dominated galaxies show wider PAH size and ionization variations.

H2 excitation temperatures are on average 200 K higher in AGN-dominated galaxies.

AGN significantly affect the molecular gas heating in host galaxies.

Abstract

We analyse 2,015 mid-infrared (MIR) spectra of galaxies observed with Spitzer's Infrared Spectrograph, including objects with growing super-massive black holes and objects where most of the infrared emission originates from newly formed stars. We determine if and how accreting super-massive black holes at the centre of galaxies -- known as active galactic nuclei (AGN) -- 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 AGN contribute more than 50 per cent of the total MIR emission as AGN-dominated. We compare the relative strengths of PAH emission features and find that PAH grains in AGN-dominated sources have a wider range of sizes and fractional ionizations than PAH grains 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. We find an average 200 K difference between the excitation temperatures of the H_2 S(5) and H_2 S(7) pure rotational molecular hydrogen transition pair in AGN-dominated versus non-AGN dominated galaxies. Our findings suggest that AGN impact the interstellar medium of their host galaxies.