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.; Ajeddig, H.; André, P.; Artis, E.; Aussel, H.; Baes, M.; Beelen, A.; Benoît, A.; Berta, S.; Bing, L.; Bourrion, O.; Calvo, M.; Casasola, V.; Catalano, A.; Chang, I. -D.; De Looze, I.; De Petris, M.; Désert, F. -X.; Doyle, S.; Driessen, E. F. C.; Ejlali, G.; Gomez, A.; Goupy, J.; Jones, A. P.; Hanser, C.; Hughes, A.; Katsioli, S.; Kéruzoré, F.; Koch, E. W.; Kramer, C.; Ladjelate, B.; Lagache, G.; Leclercq, S.; Lestrade, J. -F.; Macías-Pérez, J. F.; Maury, A.; Mauskopf, P.; Mayet, F.; Monfardini, A.; Moyer-Anin, A.; Muñoz-Echeverría, M.; Nersesian, A.; Paradis, D.; Perotto, L.; Pisano, G.; Ponthieu, N.; Revéret, V.; Rigby, A. J.; Ritacco, A.; Romero, C.; Roussel, H.; Ruppin, F.; Schuster, K.; Sievers, A.; Smith, M. W. L.; Tabatabaei, F. S.; Tedros, J.; Tucker, C.; Ysard, N.; Xilouris, E. M.; Zylka, R.

doi:10.1051/0004-6361/202557583

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 beta and the THEMIS dust model with a fixed beta. Our spatially resolved analysis was performed on similar to 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 beta, ranging from similar to 1.6 - 1.7 in diffuse regions to similar 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 about four 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 similar to 10% in the centre to similar 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 similar to 0.6 - 0.7 in star-forming regions to similar to 1.2 in the diffuse medium, consistent with cosmic ray electron ageing.

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., Ajeddig, H., André, P., Artis, E., Aussel, H., Baes, M., Beelen, A., Benoît, A., Berta, S., Bing, L., Bourrion, O., Calvo, M., Casasola, V., Catalano, A., Chang, I.-D., De Looze, I., et al.. - In: ASTRONOMY & ASTROPHYSICS. - ISSN 0004-6361. - 709:(2026), pp. 1-26. [10.1051/0004-6361/202557583]