DETECTION AND CHARACTERIZATION OF COLD INTERSTELLAR DUST AND POLYCYCLIC AROMATIC HYDROCARBON EMISSION, FROM COBE OBSERVATIONS

Using data obtained by the DIRBE instrument on the COBE spacecraft, we present the mean 3.5-240 mu m spectrum of high-latitude dust. Combine d with a spectrum obtained by the FIRAS instrument, these data represe nt the most comprehensive wavelength coverage of dust in the diffuse i nterstellar medium, spanning the 3.5-1000 mu m wavelength regime. At w avelengths shorter than similar to 60 mu m the spectrum shows an exces s of emission over that expected from dust heated by the local interst ellar radiation field and radiating at an equilibrium temperature. The DIRBE data thus extend the observations of this excess, first detecte d by the IRAS satellite at 25 and 12 mu m, to shorter wavelengths. The excess emission arises from very small dust particles undergoing temp erature fluctuations. However, the 3.5-4.9 mu m intensity ratio cannot be reproduced by very small silicate or graphite grains. The DIRBE da ta strongly suggest that the 3.5-12 mu m emission is produced by carri ers of the ubiquitous 3.3, 6.2, 7.7, 8.6, and 11.3 mu m solid state em ission features that have been detected in a wide variety of astrophys ical objects. The carriers of these features have been widely identifi ed with polycyclic aromatic hydrocarbons (PAHs). Our dust model consis ts of a mixture of PAH molecules and bare astronomical silicate and gr aphite grains with optical properties given by Draine & Lee. We obtain a very good fit to the DIRBE spectrum, deriving the size distribution , abundances relative to the total hydrogen column density, and relati ve contribution of each dust component to the observed IR emission. At wavelengths above 140 mu m the model is dominated by emission from T approximate to 17-20 K graphite and 15-18 K silicate grains. The model provides a good fit to the FIRAS spectrum in the 140-500 mu m wavelen gth regime but leaves an excess Galactic emission component at 500-100 0 mu m. The nature of this component is still unresolved. We find that (C/H) is equal to (7.3 +/- 2.2) x 10(-5) for PAHs and equal to (2.5 /- 0.8) x 10(-4) for graphite grains, requiring about 20% of the cosmi c abundance of carbon to be locked up in PAHs, and about 70% in graphi te grains [we adopt (C/H). = 3.6 x 10(-4)]. The model also requires al l of the available magnesium, silicon, and iron to be locked up in sil icates. The power emitted by PAHs is 1.6 x 10(-31) W per H atom, by gr aphite grains 3.0 x 10(-31) W per H atom, and by silicates 1.4 x 10(-3 1) W per H atom, adding up to a total infrared intensity of 6.0 x 10(- 31) W per H atom, or similar to 2 L. M.(-1). The [C II] 158 mu m line emission detected by the FIRAS provides important information on the g as phase abundance of carbon in the diffuse ISM. The 158 mu m line ari ses predominantly from the cold neutral medium (CNM) and shows that fo r typical CNM densities and temperatures C+/H = (0.5-1.0) x 10(-4), wh ich is similar to 14%-28% of the cosmic carbon abundance. The remainin g carbon abundance in the CNM, which must be locked up in dust, is abo ut equal to that required to provide the observed IR emission, consist ent with notion that most (greater than or similar to 75%) of this emi ssion arises from the neutral component of the diffuse ISM. The model provides st good fit to the general interstellar extinction curve. How ever, at UV wavelengths it predicts a larger extinction. The excess ex tinction may be the result of the UV properties adopted for the PAHs. If real, the excess UV extinction may be accounted for by changes in t he relative abundances of PAHs and carriers of the 2200 Angstrom, exti nction bump.