A 3-DIMENSIONAL DECOMPOSITION OF THE INFRARED-EMISSION FROM DUST IN THE MILKY-WAY

We have constructed a three-dimensional model of the Galactic large-sc ale infrared emission from dust associated with the molecular (H-2), n eutral atomic (H I), and extended low-density (n(e) similar to 1-100 c m(-3)) ionized (H II) gas phases of the interstellar medium. The model incorporates a three-dimensional map of the molecular and neutral ato mic hydrogen gas distributions, derived from available (CO)-C-12 and H I surveys by using the radial velocity information in the spectral li nes as a distance indicator, and available 5 and 19 GHz radio continuu m surveys to trace the column density of ionized gas. We use the model to decompose the COBE5 Diffuse Infrared Background Experiment (DIRBE) 12-240 mu m observations of the Galactic plane region (/b/less than o r equal to 5 degrees), from which the zodiacal light and stellar emiss ion have been subtracted, into distinct emission components associated with each gas phase within selected ranges of Galactocentric distance . An interstellar dust model is fitted to the resulting infrared spect ra to derive the following quantities within each Galactocentric dista nce interval: (1) the abundance and equilibrium temperature of the lar ge dust grain component within each gas phase; (2) estimates of the ab undance of very small (<200 Angstrom) transiently heated dust grains a nd polycyclic aromatic hydrocarbon (PAH) molecules; and (3) constraint s on various model parameters, such as the energy density of the ambie nt interstellar radiation held, which heats the dust within the H I ga s phase. Our results show steep negative Galactocentric gradients in t he equilibrium temperature of the large dust grain component within th e H I, H-2, and H II gas phases, the Galaxy's ambient interstellar rad iation held, and the dust-to-gas mass ratio for each gas phase. The in tensity of the ambient interstellar radiation held increases by a fact or of similar to 3 between the solar circle (8.5 kpc) and the molecula r ring at a Galactocentric distance of similar to 5 kpc. The dust abun dance gradient of (-0.05+/-0.03) dex kpc(-1) is equivalent, within the uncertainties, to the metallicity gradient in the Galactic disk. The derived emission spectra are consistent with a model in which very sma ll transiently heated dust grains and PAHs are abundant and the domina nt contributors to the mid-infrared (5 mu m<lambda<40 mu m) luminosity from a Galactocentric distance of 2 kpc out to a Galactocentric dista nce of at least 12 kpc, and indicate that the relative abundance of th e PAHs is significantly higher in the outer region of the Galactic dis k than inside the solar circle. We combine the results of our decompos ition algorithm with the results of a study of optical extinction at h igh Galactic latitude to derive the radial distribution of optical opa city in the Galactic disk and find that our Galaxy would be effectivel y transparent [A(B)(total Galaxy) <0.2 mag] to an external observer vi ewing it at a low inclination (i <30 degrees). All of the Galactic inf rared emission observed by the DIRBE can be accounted for by dust asso ciated with gas that is detected by current radio surveys, refuting th e recent suggestion that a large fraction of the dynamically inferred hidden mass in spiral galaxies may be due to unseen gas and stars in t he disk of the galaxies.