Importance of far-infrared mapping in a spiral galaxy: AKARI observation
  of M81
  The importance of the far-infrared (FIR) mapping is demonstrated for a
face-on spiral galaxy, M81, by analyzing its imaging data at 65, 90, and 140
{\mu}m taken by AKARI. Basic products are the dust temperature map, the dust
optical depth map, and the colour-colour diagram. The main features are as
follows. (i) The dust temperature derived from the total fluxes at 90 {\mu}m
and 140 {\mu}m reflects the relatively low temperatures seen in the interarm
and spiral arms excluding the warm spots, rather than the high temperatures in
warm spots and the centre. This indicates that the total FIR luminosity is
dominated by the dust heated by the general interstellar radiation field. (ii)
The galaxy is more extended at 140 {\mu}m than at the other shorter
wavelengths, which reflects the radial dust temperature gradient. (iii) The
dust optical depth derived from the FIR mapping is broadly consistent with that
estimated from the FIR-to-ultraviolet luminosity ratio. (iv) The FIR
colour-colour diagramis useful to identify a 'contamination' of warm dust. The
existence of small-scale warm star-forming regions is supported in the bright
spots along the spiral arms. This contamination also leads to an underestimate
of dust optical depth (or dust column density).
