The molecular gas properties in local Seyfert 2 galaxies^⋆

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: francesco.salvestrini@inaf.it
² Dipartimento di Astronomia “Augusto Righi”, Università degli Studi di Bologna, Viale Carlo Berti Pichat 6/2, 40127 Bologna, Italy
³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁴ ESO, Karl-Schwarzschild-Str 2, 85748 Garching bei München, Germany
⁵ INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
⁶ Department of Physics and Astronomy, Clemson University, Kinard Lab of Physics, Clemson, SC 29634, USA
⁷ Department of Space, Earth and Environment Chalmers University of Technology, 412 96 Gothenburg, Sweden
⁸ Italian ALMA Regional Centre, Via P. Gobetti 101, 40129 Bologna, Italy

Received: 26 November 2021
Accepted: 25 March 2022

Abstract

Aims. We present a multiwavelength study of the molecular gas properties of a sample of local Seyfert 2 galaxies to assess if, and to what extent, the presence of an active galactic nucleus (AGN) can affect the interstellar medium (ISM) properties in a sample of 33 local Seyfert 2 galaxies.

Methods. We compare the molecular gas content (M_H2) derived from new and archival low-J CO line measurements of a sample of AGN and a control sample of star-forming galaxies (SFGs). Both the AGN and the control sample are characterized in terms of host-galaxy properties, for example stellar and dust masses (M_⋆ and M_dust, respectively) and the star formation rate (SFR). We also investigate the effect of AGN activity on the emission of polycyclic aromatic hydrocarbon (PAH) molecules in the mid-infrared (MIR), a waveband where the dust-reprocessed emission from the obscured AGN contributes the most.

Result. The AGN hosted in less massive galaxies (i.e., M_⋆ < 10^10.5 M_⊙; M_dust < 10^7.5 M_⊙) show larger molecular gas contents with respect to SFGs that have the same stellar and dust masses. When comparing their depletion times (t_dep ∝ M_H2/SFR), AGN show t_dep ∼ 0.3 − 1.0 Gyr, similar to the times observed in the control sample of SFGs. Seyfert 2 galaxies show fainter PAH luminosity the larger the dominance of the nuclear activity in the MIR.

Conclusions. We find no clear evidence for a systematic reduction in the molecular gas reservoir at galactic scales in Seyfert galaxies with respect to SFGs. This is in agreement with recent studies that show that molecular gas content is only reduced in sub-kiloparsec-sized regions, where emission from the accreting supermassive black hole dominates. Nonetheless, we show that the impact of AGN activity on the ISM is clearly visible as a suppression of the PAH luminosity.