High-resolution, 3D radiative transfer modelling

II. The early-type spiral galaxy M 81

¹ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9 9000 Gent, Belgium
e-mail: maarten.baes@ugent.be
² National Observatory of Athens, Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, Ioannou Metaxa and Vasileos Pavlou, 15236 Athens, Greece
³ Department of Astrophysics, Astronomy & Mechanics, Faculty of Physics, University of Athens, Panepistimiopolis 15784, Zografos, Athens, Greece
⁴ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
⁵ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁶ INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 4019 Bologna, Italy
⁷ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁸ School of Physics and Astronomy, Cardiff University, Queen’s Buildings, The Parade, Cardiff CF24 3AA, UK
⁹ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
¹⁰ AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, 91191 Gif-sur-Yvette, France
¹¹ Institut d’Astrophysique Spatiale, CNRS, Université Paris-Sud, Université Paris-Saclay, Bât. 121, 91405 Orsay Cedex, France
¹² Central Astronomical Observatory of RAS, Pulkovskoye Chaussee 65/1, 196140 St. Petersburg, Russia
¹³ St. Petersburg State University, Universitetskij Pr. 28, 198504 St. Petersburg, Stary Peterhof, Russia

Received: 25 April 2019
Accepted: 7 April 2020

Abstract

Context. Interstellar dust absorbs stellar light very efficiently, thus shaping the energy output of galaxies. Studying the impact of different stellar populations on the dust heating continues to be a challenge because it requires decoupling the relative geometry of stars and dust and also involves complex processes such as scattering and non-local dust heating.

Aims. We aim to constrain the relative distribution of dust and stellar populations in the spiral galaxy M 81 and create a realistic model of the radiation field that adequately describes the observations. By investigating the dust-starlight interaction on local scales, we want to quantify the contribution of young and old stellar populations to the dust heating. We aim to standardise the setup and model selection of such inverse radiative transfer simulations so these can be used for comparable modelling of other nearby galaxies.

Methods. We present a semi-automated radiative transfer modelling pipeline that implements necessary steps such as the geometric model construction and the normalisation of the components through an optimisation routine. We used the Monte Carlo radiative transfer code SKIRT to calculate a self-consistent, panchromatic model of the interstellar radiation field. By looking at different stellar populations independently, we were able to quantify to what extent different stellar age populations contribute to the heating of dust. Our method takes into account the effects of non-local heating.

Results. We obtained a realistic 3D radiative transfer model of the face-on galaxy M 81. We find that only 50.2% of the dust heating can be attributed to young stellar populations (≲100 Myr). We confirm that there is a tight correlation between the specific star formation rate and the heating fraction by young stellar populations, both in sky projections and in 3D, which is also found for radiative transfer models of M 31 and M 51.

Conclusions. We conclude that old stellar populations can be a major contributor to the heating of dust. In M 81, old stellar populations are the dominant heating agent in the central regions, contributing to half of the absorbed radiation. Regions of higher star formation do not correspond to the highest dust temperatures. On the contrary, it is the dominant bulge which is most efficient in heating the dust. The approach we present here can immediately be applied to other galaxies. It does contain a number of caveats, which we discuss in detail.