A great deal of attention has been devoted to the characterization of nearby spirals' IR emission. This has led to a deeper understanding of the distribution of dust in galaxies.
2.1. Cold dust and the total amount of dust
Assuming that the IRAS 60 and 100 µm fluxes from galaxies sample a single dust phase, dust masses and temperatures have been computed. Typical dust temperatures of ~ 30K and dust masses one order of magnitude below that deduced for the Milky Way were obtained (e.g. [Devereux & Young1990]). The advent of millimeter bolometers showed that this was likely an artifact of the wavelength domain sampled by IRAS, but uncertainties in the exponent of the emissivity in the FIR/mm range prevented a definitive assessment of the question.
With the combination of ISO and SCUBA, this is now
being resolved.
The full FIR/mm SEDs of nearby spiral galaxies reveal a consistent
picture: in normal galaxies (i.e. star-forming but not starbursting
spirals) most of the emission longward of 150-200 µm is
provided by a cold dust phase (T
10-20K,
[Alton et
al. 1998a];
[Bianchi et
al. 1998];
[Davies et
al. 1999];
[Haas et
al. 1998];
[Israel et
al. 1999];
[Krügel et
al. 1998]).
In fact, Alton et al. 1998a
show clearly that the cold dust
emission provides a significant fraction of the 100 µm
flux. This mixed nature of the 100 µm IRAS band is
very likely at the
origin of numerous controversies on the interpretation of IRAS
luminosities. Dust masses measurements now yield gas-to-dust
ratios in external galaxies in the range 100-300, much more in
agreement with the value of ~ 160 measured in our galaxy.
These studies also point to a FIR/mm emissivity index of 2 (i.e.
in
) rather
than the more commonly used 1.5.