M99 dust properties and photometry with NIKA2

Our spatially resolved analysis is performed on ~1.75kpc (25") scales, encompassing the galaxy centre, spiral arms, and inter-arm regions. From the MBB modelling, we find significant spatial variations in beta, 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 beta are up to a factor of ~4 higher than those derived assuming a fixed beta (1.6 on average). Variable-beta models recover expected correlations with dust-to-stellar and dust-to-gas ratios, whereas fixed-beta 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 8kpc.

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.