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 \sim 4000 quasi-independent pixels with spatial resolution of \sim 140 pc 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 \sim 20 in the center to \sim 70 in the star-forming ring at 10 kpc , consistent with the metallicity gradient .
In the 10 kpc ring the average \beta is \sim 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 10 kpc to \sim 2.5 at a radius of 3.1 kpc and then decreases to 1.7 in the center .
The dust temperature is fairly constant in the 10 kpc ring ( ranging from 17–20 K ) , but increases strongly in the bulge to \sim 30 K. Within 3.1 kpc we find the dust temperature is highly correlated with the 3.6µm flux , suggesting the general stellar population in the bulge is the dominant source of dust heating there .
At larger radii , there is a weak correlation between the star formation rate and dust temperature .
We find no evidence for ‘ dark gas ’ in M31 in contrast to recent results for the MW .
Finally , we obtained an estimate of the CO X-factor by minimising the dispersion in the gas-to-dust ratio , obtaining a value of ( 1.9 \pm 0.4 ) \times 10 ^ { 20 } cm ^ { -2 } [ K kms ^ { -1 } ] ^ { -1 } .