High-resolution, 3D radiative transfer modelling

IV. AGN-powered dust heating in NGC 1068

¹ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281, 9000 Gent, Belgium
e-mail: sebastien.viaene@ugent.be
² Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
³ National Observatory of Athens, Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, Ioannou Metaxa and Vasileos Pavlou GR-15236, Athens, Greece
⁴ Department of Astrophysics, Astronomy & Mechanics, Faculty of Physics, University of Athens, Panepistimiopolis, GR15784 Zografos, Athens, Greece
⁵ Instituto de Radioastronomía y Astrofísica, UNAM, Campus Morelia, A.P. 3-72, 58089 Morelia, Mexico
⁶ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁷ INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
⁸ INAF – Istituto di Astrofisica Spaziale e Fisica cosmica, Via A. Corti 12, 20133 Milano, Italy
⁹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA
¹⁰ School of Physics and Astronomy, Cardiff University, The Parade, Cardiff CF24 3AA, UK
¹¹ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
¹² Laboratoire AIM, CEA/DSM – CNRS – Université Paris Diderot, IRFU/Service d’Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette, France
¹³ Institut d’Astrophysique Spatiale, CNRS, Univ. Paris-Sud, Université Paris-Saclay, Bât. 121, 91405 Orsay Cedex, France
¹⁴ Central Astronomical Observatory of RAS, Pulkovskoye Chaussee 65/1, 196140 St. Petersburg, Russia
¹⁵ St. Petersburg State University, Universitetskij Pr. 28, 198504 St. Petersburg, Stary Peterhof, Russia

Received: 10 January 2020
Accepted: 30 April 2020

Abstract

The star formation rate and the mass of interstellar medium (ISM) have a high predictive power for the future evolution of a galaxy. Nevertheless, deriving such properties is not straightforward. Dust emission, an important diagnostic of star formation and ISM mass throughout the Universe, can be powered by sources unrelated to ongoing star formation. In the framework of the DustPedia project we set out to disentangle the radiation of the ongoing star formation from that of the older stellar populations. This is done through detailed 3D radiative transfer simulations of face-on spiral galaxies. We take special care in modelling the morphological features present for each source of radiation. In this particular study, we focus on NGC 1068, which in addition contains an active galactic nucleus (AGN). The effect of diffuse dust heating by an AGN (beyond the torus) has so far only been investigated for quasars. This additional dust heating source further contaminates the broadband fluxes that are used by classic galaxy modelling tools to derive physical properties. We aim to fit a realistic model to the observations of NGC 1068 and quantify the contribution of the several dust-heating sources. Our model is able to reproduce the global spectral energy distribution of the galaxy. It matches the resolved optical and infrared images fairly well, but deviates in the UV and the submillimetre (submm). This is partly due to beam smearing effects, but also because the input dust distribution is not sufficiently peaked in the centre. We find that AGN contamination of the broadband fluxes has a strong dependency on wavelength. It peaks in the mid-infrared, drops in the far-infrared, and then rises again at submm wavelengths. We quantify the contribution of the dust-heating sources in each 3D dust cell and find a median value of 83% for the star formation component. The AGN contribution is measurable at the percentage level in the disc, but quickly increases in the inner few hundred parsecs, peaking above 90%. This is the first time the phenomenon of an AGN heating the diffuse dust beyond its torus is quantified in a nearby star-forming galaxy. NGC 1068 only contains a weak AGN, meaning this effect could be stronger in galaxies with a more luminous AGN. This could significantly impact the derived star formation rates and ISM masses for such systems.