  We present new Herschel-derived images of warm dust in the Andromeda Galaxy,
M31, with unprecedented spatial resolution (~ 30 pc), column density accuracy,
and constraints on the three-dimensional distributions of dust temperature and
dust opacity index (hence grain size and composition), based on the new PPMAP
Bayesian analysis procedure. We confirm the overall radial variation of dust
opacity index reported by other recent studies, including the central decrease
within ~ 3 kpc of the nucleus. We also investigate the detailed distribution of
dust in the nuclear region, a prominent feature of which is a ~ 500 pc bar-like
structure seen previously in H{\alpha}. The nature of this feature has been the
subject of some debate. Our maps show it to be the site of the warmest dust,
with a mean line-of-sight temperature ~ 30 K. A comparison with the stellar
distribution, based on 2MASS data, provides strong evidence that it is a
gravitationally induced bar. A comparison with radial velocity maps suggests
the presence of an inflow towards the nucleus from opposite directions along
this bar, fed presumably by the nuclear spiral with which it appears to
connect. Such behaviour is common in large-scale bars in spiral galaxies, as is
the phenomenon of nested bars whereby a subkiloparsec nuclear bar exists within
a large-scale primary bar. We suggest that M31 represents an example of such
nesting.

Astrophysics of Galaxies