Spatially-resolved interstellar dust properties in the face-on spiral galaxy M 99 as observed by NIKA2
Abstract
Large dust grains in thermal equilibrium dominate the far-infrared and contribute to the millimetre continuum of star-forming galaxies, but constraining their properties is difficult due to free-free and synchrotron contamination. We study spatial variations in the dust spectral index, mass, and grain properties in the nearby face-on spiral galaxy M 99. We use new 1.15 and 2 mm continuum observations from NIKA2 on the IRAM 30 m telescope (IMEGIN Guaranteed Time Large Programme) combined with multiwavelength data from UV to radio. The infrared-to-radio SED is decomposed into dust, free-free, and synchrotron components using the hierarchical Bayesian code HerBIE. Dust is modelled via a modified blackbody (MBB) with variable millimetre spectral index beta and the THEMIS dust model with fixed beta. We perform spatially-resolved analysis at scales ~1.75 kpc (~25''), covering the centre, spiral arms, and inter-arm regions. MBB fits reveal beta variations from ~1.6-1.7 in diffuse regions to ~2.3-2.5 in dense star-forming areas, likely due to grain coagulation and changes in silicate-to-carbonaceous ratios. Dust masses from variable beta are up to ~4x higher than fixed-beta models, which systematically bias dust-to-stellar and dust-to-gas ratios. The small grain fraction rises from ~10% in the centre to ~15% in the diffuse disc, anti-correlated with the interstellar radiation field; gas-phase metallicity plays a minor role within 8 kpc. Synchrotron spectral index ranges from ~0.6-0.7 in star-forming regions to ~1.2 in the diffuse medium, consistent with cosmic-ray electron ageing.
Keywords
Cite
@article{arxiv.2602.20393,
title = {Spatially-resolved interstellar dust properties in the face-on spiral galaxy M 99 as observed by NIKA2},
author = {L. Pantoni and F. Galliano and S. C. Madden and R. Adam and P. Ade and H. Ajeddig and P. André and E. Artis and H. Aussel and M. Baes and A. Beelen and A. Benoît and S. Berta and L. Bing and O. Bourrion and M. Calvo and V. Casasola and A. Catalano and I-D. Chang and I. De Looze and M. De Petris and F. -X. Désert and S. Doyle and E. F. C. Driessen and G. Ejlali and A. Gomez and J. Goupy and A. P. Jones and C. Hanser and A. Hughes and S. Katsioli and F. Kéruzoré and E. W. Koch and C. Kramer and B. Ladjelate and G. Lagache and S. Leclercq and J. -F. Lestrade and J. F. Macías-Pérez and A. Maury and P. Mauskopf and F. Mayet and A. Monfardini and A. Moyer-Anin and M. Muñoz-Echeverría and A. Nersesian and D. Paradis and L. Perotto and G. Pisano and N. Ponthieu and V. Revéret and A. J. Rigby and A. Ritacco and C. Romero and H. Roussel and F. Ruppin and K. Schuster and A. Sievers and M. W. L. Smith and F. S. Tabatabaei and J. Tedros and C. Tucker and N. Ysard and E. M. Xilouris and R. Zylka},
journal= {arXiv preprint arXiv:2602.20393},
year = {2026}
}
Comments
17 pages, 12 figures (26 pages, 29 figures including appendices). Accepted for publication in Astronomy & Astrophysics