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Register a new userDust masses, compositions, and luminosities in the nuclear disks and the diffuse circumnuclear medium of Arp220
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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.
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We study infrared emission of 17 isolated, diffuse clouds with masses of order solar masses, to test the hypothesis that grain property variations cause the apparently low gas-to-dust ratios that have been measured in those clouds. Maps of the clouds were constructed from WISE data and directly compared to the maps of dust optical depth from Planck. The mid-infrared emission per unit dust optical depth has a significant trend toward lower values at higher optical depths. The trend can be quantitatively explained by extinction of starlight within the clouds. The relative amounts of PAH and very small grains traced by WISE, compared to large grains tracked by Planck, are consistent with being constant. The temperature of the large grains significantly decreases for clouds with larger dust optical depth; this trend is partially due to dust property variations but is primarily due to extinction of starlight. We updated the prediction for molecular hydrogen column density, taking into account variations in dust properties, and find it can explain the observed dust optical depth per unit gas column density. Thus the low gas-to-dust ratios in the clouds are most likely due to `dark gas that is molecular hydrogen.
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