Constraining Dust Properties Along the Minor Axis of Andromeda

Abstract

Previous studies of dust emission in M31 based primarily on Herschel data have reported inconsistent radial trends in the dust emissivity index ($β$), partly due to limited wavelength coverage and the intrinsic $T_{\rm dust}$-$β$ degeneracy. We aim to improve the constraints of $β$ determinations and obtain the spatial surface density and temperature distributions at GMC scales in M31 by extending the spectral coverage into the millimeter regime and by quantifying the impact of wavelength coverage on the $T_{\rm dust}$-$β$ degeneracy. We present new IRAM 30m NIKA2 1.15 mm observations along the minor axis of M31 at an angular resolution of 11 arcsec, corresponding to $\sim 40$ pc at the distance of M31. These data are used with Herschel and JCMT SCUBA-2 maps to perform pixel-by-pixel single-component modified blackbody fitting. Including the 850 $μ$m and 1.15 mm data helps reduce the $T_{\rm dust}$-$β$ degeneracy and yields an average emissivity index of $β\approx 1.8$, systematically lower than values derived from far-infrared-only analyses. Radial analysis shows higher $β$ values in the 5 kpc ring compared to the 10 kpc ring. The elevated $β$ values in the inner 5 kpc may be linked to changes in dust composition, potentially associated with enhanced starlight heating in this region. Millimeter wavelength coverage is important for improving $β$ constraints in M31. The lower $β$ values inferred from the 250--1150 $μ$m SED fitting suggest possible variations in dust emissivity and highlight the importance of sampling the long-wavelength SED slope in nearby galaxies.