The heating of dust by old stellar populations in the Bulge of M31
  We use new Herschel multi-band imaging of the Andromeda galaxy to analyze how
dust heating occurs in the central regions of galaxy spheroids that are
essentially devoid of young stars. We construct a dust temperature map of M31
through fitting modified blackbody SEDs to the Herschel data, and find that the
temperature within 2 kpc rises strongly from the mean value in the disk of 17
pm 1K to \sim35K at the centre. UV to near-IR imaging of the central few kpc
shows directly the absence of young stellar populations, delineates the radial
profile of the stellar density, and demonstrates that even the near-UV dust
extinction is optically thin in M31's bulge. This allows the direct calculation
of the stellar radiation heating in the bulge, U\ast(r), as a function of
radius. The increasing temperature profile in the centre matches that expected
from the stellar heating, i.e. that the dust heating and cooling rates track
each other over nearly two orders of magnitude in U\ast. The modelled dust
heating is in excess of the observed dust temperatures, suggesting that it is
more than sufficient to explain the observed IR emission. Together with the
wavelength dependent absorption cross section of the dust, this demonstrates
directly that it is the optical, not UV, radiation that sets the heating rate.
This analysis shows that neither young stellar populations nor stellar near-UV
radiation are necessary to heat dust to warm temperatures in galaxy spheroids.
Rather, it is the high densities of Gyr-old stellar populations that provide a
sufficiently strong diffuse radiation field to heat the dust. To the extent
which these results pertain to the tenuous dust found in the centres of
early-type galaxies remains yet to be explored.
