Spatially resolved interstellar dust properties in the face-on spiral galaxy M 99 as observed by NIKA2

¹ Department of Physics and Astronomy, Universiteit Gent, Proeftuinstraat 86 N3, B-9000 Gent, Belgium
² Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
³ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France
⁴ School of Physics and Astronomy, Cardiff University, CF24 3AA, UK
⁵ Université Grenoble Alpes, CNRS, Grenoble INP, LPSC-IN2P3, 38000 Grenoble, France
⁶ Max Planck Institute for Extraterrestrial Physics, 85748 Garching, Germany
⁷ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
⁸ Université Grenoble Alpes, CNRS, Institut Néel, Grenoble, France
⁹ Institut de RadioAstronomie Millimétrique (IRAM), Grenoble, France
¹⁰ Dipartimento di Fisica, Sapienza Università di Roma, I-00185 Roma, Italy
¹¹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹² STAR Institute, Quartier Agora – Allée du six Août, 19c, B-4000 Liège, Belgium
¹³ INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
¹⁴ Centro de Astrobiología (CSIC-INTA), Torrejón de Ardoz, 28850 Madrid, Spain
¹⁵ Institute for Research in Fundamental Sciences (IPM), School of Astronomy, Tehran, Iran
¹⁶ National Observatory of Athens, IAASARS, GR-15236 Athens, Greece
¹⁷ Faculty of Physics, University of Athens, GR-15784 Zografos, Athens, Greece
¹⁸ High Energy Physics Division, Argonne National Laboratory, Lemont, IL 60439, USA
¹⁹ Instituto de Radioastronomía Milimétrica (IRAM), Granada, Spain
²⁰ LUX, Observatoire de Paris, PSL Research Univ., CNRS, Sorbonne Univ., UPMC, 75014 Paris, France
²¹ School of Earth & Space and Department of Physics, Arizona State University, Tempe, AZ 85287, USA
²² School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
²³ INAF-Osservatorio Astronomico di Cagliari, 09047 Selargius, Italy
²⁴ LPENS, ENS, PSL Research Univ., CNRS, Sorbonne Univ., Université de Paris, 75005 Paris, France
²⁵ Department of Physics and Astronomy, University of Pennsylvania, PA 19104, USA
²⁶ Institut d’Astrophysique de Paris, CNRS (UMR7095), 75014 Paris, France
²⁷ University of Lyon, UCB Lyon 1, CNRS/IN2P3, IP2I, 69622 Villeurbanne, France
²⁸ Institut d’Astrophysique Spatiale (IAS), CNRS, Université Paris Sud, Orsay, France
²⁹ IRAP, Université de Toulouse, CNRS, UPS, IRAP, Toulouse Cedex 4, France
³⁰ Centre for Astrophysics – Harvard & Smithsonian, 60 Garden Street, 02138 Cambridge, MA, USA
³¹ National Radio Astronomy Observatory, 800 Bradbury SE, Suite 235 Albuquerque, NM 87106, USA
³² Institute of Astronomy and Astrophysics, Academia Sinica, No. 1, Sec. 4, Roosevelt Road, Taipei 106319, Taiwan

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Received: 7 October 2025
Accepted: 15 February 2026

Abstract

Context. Large dust grains in thermal equilibrium dominate the far-infrared emission of star-forming galaxies and substantially contribute to their millimetre continuum. Constraining dust properties in this regime is challenging due to contamination from free-free and synchrotron emission.

Aims. We investigate the spatial variations in the dust spectral index, dust mass, and grain size and composition in the nearby face-on spiral galaxy M 99. To this end, we used new 1.15 and 2 mm continuum observations obtained with NIKA2 on the IRAM 30 m telescope as part of the IMEGIN Guaranteed Time Large Programme combined with ancillary data spanning ultraviolet to radio wavelengths.

Methods. We decomposed the infrared-to-radio spectral energy distribution of M 99 into dust, free-free, and synchrotron components using the hierarchical Bayesian spectral energy distribution fitting code HerBIE. We modelled the dust emission using both a modified blackbody (MBB) with a variable millimetre spectral index β and the THEMIS dust model with a fixed β. Our spatially resolved analysis was performed on ∼1.75 kpc (25″) scales, encompassing the galaxy centre, spiral arms, and inter-arm regions.

Results. From the MBB modelling, we found significant spatial variations in β, ranging from ∼1.6 − 1.7 in diffuse regions to ∼2.3 − 2.5 in denser star-forming environments. These variations likely reflect dust grain evolution driven by coagulation and changes in the silicate-to-carbonaceous grain abundance. Dust masses inferred with variable β are up to a factor of about four higher than those derived assuming a fixed β (1.6 on average). Variable-β models recover expected correlations with dust-to-stellar and dust-to-gas ratios, whereas fixed-β models systematically bias these quantities. The small grain fraction increases from ∼10% in the centre to ∼15% in the diffuse disc and is anti-correlated with the interstellar radiation field intensity, while gas-phase metallicity plays only a minor role within the central 8 kpc. The synchrotron spectral index varies from ∼0.6 − 0.7 in star-forming regions to ∼1.2 in the diffuse medium, consistent with cosmic ray electron ageing.