Max-Planck-Institut für Astronomie, Heidelberg, Germany
A complete 175
map of the Andromeda galaxy (M31) at 1 arcmin
resolution shows
the distribution of cold dust. It is dominated by a ring at 10 kpc radius
supplemented by a faint outer one at 14 kpc. No clear spiral pattern is
recognisable. The azimuthally averaged radial brightness profile is rather flat
within the 10 kpc ring and decreases exponentially outside thereof, discernible
down to a brightness of 0.07 MJy/sr at a distance of 22 kpc.
Since the ring comprises a large reservoir for star formation, as an
evolutionary conjecture M31 might be in a transition phase changing its
classical optical Sb type spiral morphology towards that of a ringed galaxy.
The bulk of the dust has a temperature of only 16 K, considerably colder than
the 21-22 K previously inferred from the IRAS data and also colder than the 19
K
found for the Milky Way. The cold dust is accompanied by warm dust, formally
described by a component at about 45 K.
At the common resolution of 2.5 arcmin the triplet 60/100/175
flux
ratio
varies only little across the rings as well as the disk, thus everywhere in M31
at least two dust components are required to fit the far-infrared spectral
energy distribution.
This provides a direct evidence in M31 for the existence of two dust
populations
- small and large grains - similar to what had been found in the Milky Way.
For the cold dust component around 16 K we can now estimate the corresponding
mass from its emission yielding
,
a dust mass about a factor of ten
higher than inferred from the IRAS 60/100um data alone.
The new cold dust mass - if evenly distributed in the plane of the galaxy -
would be sufficient to make the disk of M31 moderately opaque in the optical
(face-on:
corresponding to
).