Letter to the Editor

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

¹ Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
e-mail: igbernete@gmail.com
² Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
³ Centro de Astrobiología, CSIC-INTA, Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁴ Observatorio Astronómico Nacional (OAN-IGN)-Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain
⁵ Telespazio UK for the European Space Agency (ESA), ESAC, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Spain
⁶ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁷ Instituto de Astrofísica de Canarias, Calle Vía Láctea, s/n, 38205 La Laguna, Tenerife, Spain
⁸ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁹ Max-Planck-Institut fur extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
¹⁰ Department of Physics & Astronomy, University of Southampton, Hampshire, SO17 1BJ Southampton, UK
¹¹ School of Mathematics, Statistics and Physics, Newcastle University, Herschel Building, Newcastle upon Tyne NE1 7RU, UK
¹² Centre for Extragalactic Astronomy, Durham University, South Road, Durham DH1 3LE, UK
¹³ Departamento de Física de la Tierra y Astrofísica, Fac. de CC Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹⁴ Instituto de Física de Partćulas y del Cosmos IPARCOS, Fac. CC Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹⁵ Department of Physics & Astronomy, University of Alaska Anchorage, AK 99508-4664, USA
¹⁶ The University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, USA

Received: 24 August 2022
Accepted: 20 September 2022

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 (SF) regions. This study represents the first of its kind to use sub-arcsecond angular resolution data of local luminous Seyferts (L_bol >  10^44.46 erg s⁻¹) with a wide wavelength coverage (4.9–28.1 μm). 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 of luminous Seyfert galaxies (∼0.45″ at 12 μm; in the inner ∼142–245 pc). We find that the nuclear regions of active galactic nuclei (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 the AGN-dominated galaxy NGC 6552 indicate the presence of larger PAH molecules compared with those of the SF regions. Therefore, our results provide evidence that the AGN have a significant impact on the ionisation state (and probably the size) of the PAH grains on scales of ∼142–245 pc.