The Impact of an AGN on PAH Spectral Features in Galaxy Evolution Simulations: Continuum Dilution and Excitation of PAHs

Galaxies hosting an active galactic nucleus (AGN) show diminished power and non-standard spectral feature ratios from polycyclic aromatic hydrocarbons (PAHs) compared with normal star-forming galaxies. Proposed mechanisms for this difference include a combination of AGN-driven PAH grain processing, differential heating from the surrounding radiation field, and continuum dilution. These potentially competing factors are difficult to disentangle observationally. In this paper, we investigate the impact of the AGN radiation field on the excitation properties and potential dilution of the mid-IR emission features via constrained numerical experiments of idealized galaxies in simulations that include a self-consistent model for the evolution of dust. We find that, for the integrated spectrum of our simulated Milky Way analog and independent of grain-processing physics, the apparent PAH emission is progressively diminished as AGN luminosity is increased. For spectra within the central half kiloparsec, PAH features are entirely obfuscated when an AGN of any sampled luminosity is present. We additionally find that the UV radiation from an AGN is capable of impacting PAH band ratios, though note that we do not yet include a model for potential photodestruction of PAHs. Our model suggests that the inferred lack of PAHs via featureless mid-IR spectra from galaxies hosting AGN may simply owe to the direct continuum from the AGN overpowering mid-IR features from PAHs in the ISM of the host galaxy. Future work will investigate the relative role of AGN continuum dilution and PAH heating vs photodestruction/ionization of PAHs.