Detection and Characterization of Cold Interstellar Dust and PAH Emission from COBE Observations

Using data obtained by the DIRBE and FIRAS instruments on the COBE spacecraft we present the mean 3.5-1000 um dust spectrum from the high latitude cirrus clouds. These data represent the most comprehensive wavelength coverage of dust emission, and first strong evidence for the presence of PAHs in cirrus. The COBE data are well fit with a dust model consisting of a mixture of PAH molecules and bare astronomical silicate and graphite grains. From the model we derive the size distribution, abundances relative to the total hydrogen column density, and relative contribution of each dust component to the observed IR emission. The model provides a good fit to the FIRAS spectrum in the 140-500 um wavelength regime, but leaves an excess Galactic emission component at 500-1000 um. The nature of this component is still unresolved. The model requires about 85% of the solar carbon abundance to be locked up in dust, a third of that in PAHs. The remaining ~ 15% is in the gas as inferred from the [C II] 158 um line emission detected by the FIRAS. PAHs emit about 25% of the total IR power, and must therefore be responsible for the same fraction of the total UV-optical extinction.