Multi-temperature mapping of dust structures throughout the Galactic
  Plane using the PPMAP tool with Herschel Hi-GAL data
  We describe new Hi-GAL based maps of the entire Galactic Plane, obtained
using continuum data in the wavelength range 70-500 $\mu$m. These maps are
derived with the PPMAP procedure, and therefore represent a significant
improvement over those obtained with standard analysis techniques. Specifically
they have greatly improved resolution (12 arcsec) and, in addition to more
accurate integrated column densities and mean dust temperatures, they give
temperature-differential column densities, i.e., separate column density maps
in twelve distinct dust temperature intervals, along with the corresponding
uncertainty maps. The complete set of maps is available on-line. We briefly
describe PPMAP and present some illustrative examples of the results. These
include (a) multi-temperature maps of the Galactic HII region W5-E, (b) the
temperature decomposition of molecular cloud column-density probability
distribution functions, and (c) the global variation of mean dust temperature
as a function of Galactocentric distance. Amongst our findings are: (i) a
strong localised temperature gradient in W5-E in a direction orthogonal to that
towards the ionising star, suggesting an alternative heating source and
providing possible guidance for models of the formation of the bubble complex,
and (ii) the overall radial profile of dust temperature in the Galaxy shows a
monotonic decrease, broadly consistent both with models of the interstellar
radiation field and with previous estimates at lower resolution. However, we
also find a central temperature plateau within ~ 6 kpc of the Galactic centre,
outside of which is a pronounced steepening of the radial profile. This
behaviour may reflect the greater proportion of molecular (as opposed to
atomic) gas in the central region of the Galaxy.
