The Ionization and Destruction of PAHs in AGNs

Polycyclic aromatic hydrocarbon (PAH) emission serves as a valuable tracer of star formation rate (SFR) and a diagnostic of the physical conditions of the interstellar medium in galaxies. PAH emission is detected in galaxies containing active galactic nuclei (AGNs), both in the integrated mid-infrared (IR) spectra and from spatially resolved spectra of nearby galaxies. However, their applicability to the host galaxies of AGNs as an SFR indicator is controversial given the ability of PAH molecules to survive the harsh radiation field of an active nucleus. Many works have shown that AGNs present distinctive PAH band ratios and mass content from the star-forming galaxies. In this talk, I present our recent study on PAH emissions of a lower-redshift quasar sample. We find that PAHs increasingly underestimate the SFR (by ∼0.5 dex) for quasars with bolometric luminosity > 1046 ergs/s. Furthermore, the PAH ratios indicate that powerful AGNs preferentially destroy small grains and enhance the PAH ionization fraction in their host galaxies. In the end, I discuss how the next-generation observations from the space and ground telescope will advance our understanding of this field.
The content of the talk is published in Astrophysical Journal, 925, 218, 2022, with the DOI: https://doi.org/10.3847/1538-4357/ac32e2