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

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.