A high angular resolution view of the PAH emission in Seyfert galaxies using JWST/MRS data

TL;DR

This study uses high-resolution JWST/MRS data to analyze PAH emission in Seyfert galaxies, revealing differences in PAH properties between nuclear and star-forming regions and the influence of AGN on PAH ionization and size.

Contribution

First high-resolution analysis of nuclear PAH emission in Seyferts using JWST, comparing PAH properties with star-forming regions and modeling their ionization states.

Findings

Nuclear PAH features are present within ~142-245 pc of Seyferts.

AGN regions show different PAH ratios compared to star-forming regions.

AGN influence PAH ionization and size, favoring neutral and larger PAHs.

Abstract

Polycyclic Aromatic Hydrocarbons (PAHs) are carbon-based molecules that are ubiquitous in a variety of astrophysical objects and environments. In this work, we use JWST/MIRI MRS spectroscopy of three Seyferts to compare their nuclear PAH emission with that of star-forming regions. This study represents the first of its kind using sub-arcsecond angular resolution data of local luminous Seyferts (Lbol>10^44.46 erg/s) on a wide wavelength coverage (4.9-28.1 micron). We present an analysis of their nuclear PAH properties by comparing the observed ratios with PAH diagnostic model grids, derived from theoretical spectra. Our results show that a suite of PAH features is present in the innermost parts (~0.45 arcsec at 12 micron; in the inner ~142-245 pc) of luminous Seyfert galaxies. We find that the nuclear regions of AGN lie at different positions of the PAH diagnostic diagrams, whereas the SF regions are concentrated around the average values of SF galaxies. In particular, we find that the nuclear PAH emission mainly originates in neutral PAHs. In contrast, PAH emission originating in the SF regions favours ionised PAH grains. The observed PAH ratios in the nuclear region of AGN-dominated galaxy NGC 6552 indicate the presence of larger-sized PAH molecules compared with those of the SF regions. Therefore, our results provide evidence that the AGN have a significant impact on the ionization state (and probably the size) of the PAH grains on scales of ~142-245 pc.