While warm dust at temperatures about 30K was well detected in normal galaxies
by IRAS and the ISO data obtained so far, cold dust at temperatures about 15K
was not yet found. Possibly it is too difficult to detect this second dust
component within a dust mixture in which the radiation of warm dust dominates.
We propose to search for the cold dust component in a normal galaxy by choosing
appropriate, spatially resolved regions and applying optimal instrumental
configurations.
We would like to perform a long scan (AOT P22) right across the southwestern
part of the andromeda galaxy M31 using the C200 detector of ISOPHOT with the
FIR filters at 120, 135, 180 and 200 micron (centered at 120, 150, 185 and 205
micron). The chosen wavelength regime between 100 and 240 micron is optimal for
the investigation of dust at temperatures about 15-30K. It makes full usage of
the longer wavelength capabilities of ISOPHOT.
The scan hits the ringlike region of relatively warm dust which surrounds the
disk. At the same time the scan goes through the inner regions southwest of the
bulge with very low 60 and 100 micron radiation, i.e. essentially free of warm
dust. The existence of dusty material, however, is indicated for example by
Hodge (1980, AJ 85, 376) who found several hundred dark clouds from
investigations of photoplates. This inner region seems to be an ideal location
to search for cold dust. The colors derived from the ringlike regions allow a
direct differential comparison.