Dust in the diffuse ISM as revealed by DIRBE observations

The weekly averaged DIRBE full sky images have been processed to separate the various components contributing to the total brightness in the various bands. The zodiacal emission, which dominates at 12 and 25 #m and the zodiacal dust scattering at ~ < 5 #m are both accounted for using an empirical fit to the data. The diffuse stellar emission which dominates at )~ < 5 #m is determined using the shortest DIRBE photometric bands at 1.25 and 2.2 #m and a standard NIR extinction law. Preliminary results based on the first release of the DIRBE data have been presented in Bernard et al. 1994. When the zodiacal light and stellar emission are subtracted, significant emission remains above 2.2 #m, which follows the general distribution of the dust emission as seen in the IRAS bands. The DIRBE images therefore allow to extend our knowledge of the dust emission spectrum below 12 #m and above 100 #m. In the L(3.5 #m) and M(4.9 #m) bands, the dust emission can be seen not only toward the galactic plane but also in diffuse regions above the plane as well as toward closeby molecular complexes (p-Ophiuchi, Orion, Taurus,...). The existence of NIR dust emission in cold and diffuse regions strongly suggests transiently heated small dust particles as the carrier. The dust NIR spectrum is generally consistent with the dust model of D~sert et al. 1990. In particular, the dust emission increases from 4.9 to 3.5 #m, which can be attributed to the contribution of the 3.3 #m emission feature of Polycyclic Aromatic Hydrocarbons (PAH). Significant continuum emission, or other feature emission, is also required to explain the observed brightness in the L band and the AROME ballon experiment results at low galactic latitude.