Interstellar dust properties of M51 from AKARI mid-infrared images
  Using mid-infrared (MIR) images of four photometric bands of the Infrared
Camera (IRC) onboard the AKARI satellite, S7 (7 um), S11 (11 um), L15 (15 um),
and L24 (24 um), we investigate the interstellar dust properties of the nearby
pair of galaxies M51 with respect to its spiral arm structure. The arm and
interarm regions being defined based on a spatially filtered stellar component
model image, we measure the arm-to-interarm contrast for each band. The
contrast is lowest in the S11 image, which is interpreted as that among the
four AKARI MIR bands the S11 image best correlates with the spatial
distribution of dust grains including colder components, while the L24 image
with the highest contrast traces warmer dust heated by star forming activities.
The surface brightness ratio between the bands, i.e. color, is measured over
the disk of the main galaxy, M51a, at 300 pc resolution. We find that the
distribution of S7/S11 is smooth and well traces the global spiral arm pattern
while L15/S11 and L24/S11 peak at individual HII regions. This result indicates
that the ionization state of PAHs is related to the spiral structure.
Comparison with observational data and dust models also supports the importance
of the variation in the PAH ionization state within the M51a disk. However, the
mechanism driving this variation is not yet clear from currently available data
sets. Another suggestion from the comparison with the models is that the PAH
fraction to the total dust mass is higher than previously estimated.
