Mid-Infrared Spectroscopic Evidence for AGN Heating Warm Molecular Gas

TL;DR

This study analyzes 2,015 Spitzer/IRS mid-infrared spectra to quantify how AGN heating alters the warm molecular gas and dust in host galaxies. Using PAH EQW to separate AGN- and SF-dominated MIR emission and multiple H2 diagnostics, the authors demonstrate elevated H2 emission and higher excitation temperatures in AGN hosts, supported by both two-temperature decompositions and hierarchical Bayesian modelling. The results reveal a clear AGN-linked excess in H2 relative to PAH emission and a statistically distinct warmer H2 component, implying AGN energy couples to the ISM and heats molecular gas. These findings advance our understanding of AGN feedback effects on the ISM across a large, diverse galaxy sample and highlight the value of combining MIR spectroscopy with Bayesian inference for ISM studies.

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.

Figures (18)

• Figure 1: Quality of the sample via CASSIS pipeline: The detection level is the maximal average signal-to-noise ratio among the module/order/nod spectra, and is a data product provided with the reduced spectra. We only use spectra with detection levels $\ge 3\sigma$ and accurate redshift determination, which is discussed in \ref{['sec:sampprop']}. This leaves us with 2,015 objects.
• Figure 2: Spectrophotometry Test: In the leftmost plot, the filled black circles and the empty green circles are the 22 and 12 $\upmu\mathrm m$WISE bandpasses respectively. We calculate the synthetic WISE photometry by convolving the observed IRS spectrum with the relevant WISE bandpass transmission curve. We plot an orange line with a slope of one for visual reference. In the rightmost plots, we show the distribution of the difference of the observed to synthetic photometry for each band. The median differences between the WISE synthetic and observed 12 and 22 $\upmu\mathrm m$ bands are $0.11$ and $-0.10$ mag respectively.
• Figure 3: Redshift distribution from the cross-matched IDEOS objects. The median and mean redshifts for our sample are 0.15 and 0.4 respectively.
• Figure 4: $K_s$-band luminosity distribution for the objects in our sample with $z < 0.05$ derived using 2MASS $K_s$-band. We show the luminosity distributions of selected galaxy sub-samples with maddox for reference.
• Figure 5: Stacked Spitzer spectra binned by EQW[PAH 6.2 $\upmu\mathrm m$]: we split our sample into 100 evenly populated bins of EQW[PAH 6.2 $\upmu\mathrm m$]. We normalize each pre-stacked individual spectrum by its IRS $f_{\nu}$[24 $\upmu\mathrm m$]. We use the blue-to-yellow gradient colormap throughout this work to indicate the EQW[PAH 6.2 $\upmu\mathrm m$], with blue corresponding to AGN-dominated and yellow SF-dominated. The inset shows only two spectra from the stacks, a low EQW (blue) and high EQW (yellow) stack, and is meant to provide an easy comparison between the stacks. We provide the entirety of the stacked spectra in ASCII format in the on-line version of this publication.