The Herschel Exploitation of Local Galaxy Andromeda (HELGA) II: Dust and Gas in Andromeda
M. W. L. Smith, S. A. Eales, H. L. Gomez, J. Roman Duval, J. Fritz, R., Braun, M. Baes, G. J. Bendo, J. A. D. L Blommaert, M. Boquien, A. Boselli, D., L. Clements, A. R. Cooray, L. Cortese, I. de Looze, G. P. Ford, W. K. Gear,, G. Gentile, K. D. Gordon, J. Kirk, V. Lebouteiller

TL;DR
This study analyzes the distribution and properties of dust and gas in Andromeda using Herschel infrared data, revealing radial variations in dust emissivity, temperature, and gas-to-dust ratio, with no evidence of cold or dark gas components.
Contribution
It provides detailed spatially-resolved measurements of dust and gas properties in Andromeda, including variable dust emissivity index and gas-to-dust ratio, and estimates the CO X-factor.
Findings
Radial variation in dust emissivity index (beta) from 1.9 to 2.5 and back to 1.7.
No significant cold dust or dark gas detected in Andromeda.
Gas-to-dust ratio increases from ~20 to ~70 from center to 10kpc.
Abstract
We present an analysis of the dust and gas in Andromeda, using Herschel images sampling the entire far-infrared peak. We fit a modified-blackbody model to ~4000 quasi-independent pixels with spatial resolution of ~140pc and find that a variable dust-emissivity index (beta) is required to fit the data. We find no significant long-wavelength excess above this model suggesting there is no cold dust component. We show that the gas-to-dust ratio varies radially, increasing from ~20 in the center to ~70 in the star-forming ring at 10kpc, consistent with the metallicity gradient. In the 10kpc ring the average beta is ~1.9, in good agreement with values determined for the Milky Way (MW). However, in contrast to the MW, we find significant radial variations in beta, which increases from 1.9 at 10kpc to ~2.5 at a radius of 3.1kpc and then decreases to 1.7 in the center. The dust temperature is…
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