Cold molecular gas and PAH emission in the nuclear and circumnuclear regions of Seyfert galaxies

¹ Centro de Astrobiología (CAB, CSIC-INTA), ESAC Campus, Villanueva de la Cañada, 28692 Madrid, Spain
e-mail: aalonso@cab.inta-csic.es
² Centro de Astrobiología (CAB, CSIC-INTA), Carretera de Torrejón a Ajalvir, Torrejón de Ardoz, 28880 Madrid, Spain
³ Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
⁴ Observatorio de Madrid, OAN-IGN, Alfonso XII, 3, 28014 Madrid, Spain
⁵ LERMA, Obs. de Paris, PSL Research Univ., Collège de France, CNRS, Sorbonne Univ., UPMC, Paris, France
⁶ Max Planck Institut fuer extraterrestrische Physik Postfach 1312, 85741 Garching bei München, Germany
⁷ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Ejército Libertador 441, Santiago 8320000, Chile
⁸ Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100101, China
⁹ Instituto de Radioastronomía y Astrofísica (IRyA-UNAM), 3-72 (Xangari), 8701 Morelia, Mexico
¹⁰ Centro de Estudios de Física del Cosmos de Aragón, Unidad Asociada al CSIC, Plaza San Juan 1, 44001 Teruel, Spain
¹¹ Department of Physics & Astronomy, University of Alaska Anchorage 99508-4664, USA
¹² Department of Physics & Astronomy, University of Southampton, Hampshire, SO17 1BJ Southampton, UK
¹³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁴ Instituto de Astrofísica de Canarias, Calle vía Láctea, s/n, 38205 La Laguna, Tenerife, Spain
¹⁵ Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
¹⁶ Centre for Extragalactic Astronomy, Durham University, South Road, Durham DH1 3LE, UK

Received: 3 February 2020
Accepted: 5 May 2020

Abstract

We investigate the relation between the detection of the 11.3 μm polycyclic aromatic hydrocarbon (PAH) feature in the nuclear (∼24−230 pc) regions of 22 nearby Seyfert galaxies and the properties of the cold molecular gas. For the former we use ground-based (0.3−0.6″ resolution) mid-infrared (mid-IR) spectroscopy. The cold molecular gas is traced by ALMA and NOEMA high (0.2−1.1″) angular resolution observations of the CO(2–1) transition. Galaxies with a nuclear detection of the 11.3 μm PAH feature contain more cold molecular gas (median 1.6 × 10⁷ M_⊙) and have higher column densities (N(H₂) = 2 × 10²³ cm⁻²) over the regions sampled by the mid-IR slits than those without a detection. This suggests that molecular gas plays a role in shielding the PAH molecules in the harsh environments of Seyfert nuclei. Choosing the PAH molecule naphthalene as an illustration, we compute its half-life in the nuclear regions of our sample when exposed to 2.5 keV hard X-ray photons. We estimate shorter half-lives for naphthalene in nuclei without a 11.3 μm PAH detection than in those with a detection. The Spitzer/IRS PAH ratios on circumnuclear scales (∼4″ ∼ 0.25−1.3 kpc) are in between model predictions for neutral and partly ionized PAHs. However, Seyfert galaxies in our sample with the highest nuclear H₂ column densities are not generally closer to the neutral PAH tracks. This is because in the majority of our sample galaxies, the CO(2–1) emission in the inner ∼4″ is not centrally peaked and in some galaxies traces circumnuclear sites of strong star formation activity. Spatially resolved observations with the MIRI medium-resolution spectrograph on the James Webb Space Telescope will be able to distinguish the effects of an active galactic nucleus (AGN) and star formation on the PAH emission in nearby AGN.