The interstellar cold dust observed by COBE

Using DIRBE and FIRAS maps at high latitude ($|b|>10^{\circ}$) we derive the spatial distribution of the dust temperature associated with the diffuse cirrus and the dense molecular clouds. For a $\nu^2$ emissivity law, we find that the equilibrium dust temperature of the cirrus is about 17.5 K with only small variations over the high latitude sky. Comparison of the far Infrared DIRBE maps shows the presence of a colder emission component with a temperature around 15 K, assuming a $\nu^2$ emissivity law. The lowest values of the temperature found in the cold regions ($\sim 13 K$) are compatible with the results recently obtained for dense cores in star forming regions by the balloon-borne experiment SPM-PRONAOS (Ristorcelli et al., 1996, 1998, Serra et al., 1997). This cold component is in particular present in the direction of known molecular complexes with low star forming activity such as Taurus. The association between the cold component and molecular clouds is further demonstrated by the fact that all sky pixels with significant cold emission have an excess IR emission with respect to the high latitude IR/HI correlation. We have deduced a threshold value of the column density, $N_{HI} = 2.5 10^{20} H/cm^2$, below which cold dust is not detected within the FIRAS beam of $\sim 7\degr$. We have re-examined the problem of the existence of a very cold dust component ($T \sim 7 K$) by combining DIRBE maps of the cold emission with FIRAS spectra, corrected for the isotropic component found in Puget et al. (1996). The warm and cold component deduced from the analysis of DIRBE maps account for the Galactic FIRAS spectra with no need for a very cold component ($T\sim7$ K).