Detection and Characterization of Cold Interstellar Dust and Polycyclic Aromatic Hydrocarbon Emission, from COBEObservations

Using data obtained by the DIRBE instrument on the COBEspacecraft, we present the mean 3.5-240 ?m spectrum of high-latitude dust. Combined with a spectrum obtained by the FIRAS instrument, these data represent the most comprehensive wavelength coverage of dust in the diffuse interstellar medium, spanning the 3.5-1000 ?m wavelength regime. At wavelengths shorter than ~60 ?m the spectrum shows an excess of emission over that expected from dust heated by the local interstellar radiation field and radiating at an equilibrium temperature. The DIRBE data thus extend the observations of this excess, first detected by the IRASsatellite at 25 and 12 ?m, to shorter wavelengths. The excess emission arises from very small dust particles undergoing temperature fluctuations. However, the 3.5-4.9 ?m intensity ratio cannotbe reproduced by very small silicate or graphite grains. The DIRBE data strongly suggest that the 3.5-12 ?m emission is produced by carriers of the ubiquitous 3.3, 6.2, 7.7, 8.6, and 11.3 ?m solid state emission features that have been detected in a wide variety of astrophysical objects. The carriers of these features have been widely identified with polycyclic aromatic hydrocarbons (PAHs).