Dust masses, compositions, and luminosities in the nuclear disks and the
  diffuse circumnuclear medium of Arp220
  We present an analysis of the 4-2600 $\mu$m spectral energy distributions
(SEDs) of the west and east nuclei and the diffuse infrared (IR) region of the
merger-driven starburst Arp 220. We examine several possible source
morphologies and dust temperature distributions using a mixture of silicate and
carbonaceous grains. From fits to the SEDs we derive dust masses, temperatures,
luminosities, and dust inferred gas masses. We show that the west and east
nuclei are powered by central sources deeply enshrouded behind $\sim 10^{26}$
cm$^{-2}$ column densities of hydrogen with an exponential density
distribution, and that the west and east nuclei are optically thick out to
wavelengths of $\sim 1900$ and $\sim 770$ $\mu$m, respectively. The nature of
the central sources cannot be determined from our analysis. We derive star
formation rates or black hole masses needed to power the IR emission, and show
that the [C II] 158$\mu$m line cannot be used as a tracer of the star formation
rate in heavily obscured systems. Dust inferred gas masses are larger than
those inferred from CO observations, suggesting either larger dust-to-H mass
ratios, or the presence of hidden atomic H that cannot be inferred from CO
observations. The luminosities per unit mass in the nuclei are $\sim 450$, in
solar units, smaller that the Eddington limit of $\sim 1000 - 3000$ for an
optically thick star forming region, suggesting that the observed gas outflows
are primarily driven by stellar winds and supernova shock waves instead of
radiation pressure on the dust.
