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

Pantoni, L., Galliano, F., Madden, S. C., Adam, R., Ade, P. ORCID: https://orcid.org/0000-0002-5127-0401, Ajjeddig, H., Doyle, S. ORCID: https://orcid.org/0000-0002-9054-986X and Tucker, C. ORCID: https://orcid.org/0000-0002-1851-3918 2026. Spatially resolved interstellar dust properties in the face-on spiral galaxy M 99 as observed by NIKA2. Astronomy & Astrophysics 709 , A1. 10.1051/0004-6361/202557583

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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.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Schools > Physics and Astronomy Schools > Physical, Chemical & Environmental Sciences
Publisher:	EDP Sciences
ISSN:	0004-6361
Date of First Compliant Deposit:	8 October 2026
Date of Acceptance:	15 February 2026
Last Modified:	08 Oct 2026 15:30
URI:	https://orca.cardiff.ac.uk/id/eprint/190108

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