LARGE-SCALE CHARACTERISTICS OF INTERSTELLAR DUST FROM COBE DIRBE OBSERVATIONS

Observations from the COBE Diffuse infrared Background Experiment of the 140 and 240 mum emission from the Galactic plane region (Absolute value of b < 10-degrees) are combined with radio surveys that trace the molecular (H-2), neutral atomic (H I), and extended low-density (n(e) approximately 10-100 cm-3) ionized (H II) gas phases of the interstellar medium to derive physical conditions such as the dust temperature, dust-to-gas mass ratio, and far-infrared emissivity (1) averaged over these gas phases along each line of sight and (2) within each of these three gas phases. This analysis shows large-scale longitudinal and latitudinal gradients in the dust temperature and a decrease in dust temperature with increasing Galactocentric distance. The derived dust temperatures are significantly different from those derived in similar analyses using the Infrared Astronomical Satellite (IRAS) 60 and 100 mum data, suggesting that small (5 angstrom less than or similar to radius less than or similar to 200 angstrom) transiently heated dust particles contribute significantly to the Galactic 60 mum emission. It is found that 60%-75% of the far-infrared luminosity arises from cold (approximately 17-22 K) dust associated with diffuse H I clouds, 15%-30% from cold (approximately 19 K) dust associated with molecular gas, and less than 10% from warm (approximately 29 K) dust in extended low-density H II regions, consistent with the results of the IRAS analyses of the Galactic 60 and 100 mum emission. Within 2-degrees of longitude of the Galactic center, the derived gas-to-dust mass ratio along the line of sight, G(d), reverses its general trend of decreasing G(d) toward the inner Galaxy and increases by a factor of approximately 2-3 toward the Galactic center. One possible explanation for this result is that the ratio of H-2 column density to (CO)-C-12 intensity is lower in the Galactic center region than in the Galactic disk.