THE INTERSTELLAR COLD DUST OBSERVED BY COBE

Using DIRBE and FIRAS maps at high latitude (/b/ > 10 degrees) we deri ve the spatial distribution of the dust temperature associated with th e diffuse cirrus and the dense molecular clouds. For a nu(2) emissivit y law, we fmd that the equilibrium dust temperature of the cirrus is a bout 17.5 K with only small variations over the high latitude sky. Com parison of the far Infrared DIRBE maps shows the presence of a colder emission component with a temperature around 15 K, assuming a nu(2) em issivity law. The lowest values of the temperature found in the cold r egions (similar to 13 K) are compatible with the results recently obta ined for dense cores in star forming regions by the balloon-borne expe riment SPM-PRONAOS (Ristorcelli et al., 1996, in prep., Serra et al., 1997). This cold component is in particular present in the direction o f known molecular complexes with low star forming activity such as Tau rus. The association between the cold component and molecular clouds i s further demonstrated by the fact that all sky pixels with significan t cold emission have an excess IR emission with respect to the high la titude IR/HI correlation. We have deduced a threshold value of the col umn density, N-HI=2.5 10(20) H cm(-2), below which cold dust is not de tected within the FIRAS beam of similar to 7 degrees. We have re-exami ned the problem of the existence of a very cold dust component (T simi lar to 7 K) by combining DIRBE maps of the cold emission with FIRAS sp ectra, corrected for the isotropic component found in Puget et al. (19 96). The warm and cold component deduced from the analysis of DIRBE ma ps account for the Galactic FIRAS spectra with no need for a very cold component (T similar to 7 K).