Extreme dust disks in Arp 220 as revealed by ALMA
  We present new images of Arp 220 from the Atacama Large
Millimeter/submillimeter Array with the highest combination of frequency (691
GHz) and resolution ($0.36 \times 0.20^{\prime\prime}$) ever obtained for this
prototypical ultraluminous infrared galaxy. The western nucleus is revealed to
contain warm (200 K) dust that is optically thick ($\tau_{434\mu m} = 5.3$),
while the eastern nucleus is cooler (80 K) and somewhat less opaque
($\tau_{434\mu m} = 1.7$). We derive full-width half-maximum diameters of $ 76
\times \le 70$ pc and $123 \times 79$ pc for the western and eastern nucleus,
respectively. The two nuclei combined account for ($83 ^{+65}_{-38}$
(calibration) $^{+0}_{-34}$ (systematic))% of the total infrared luminosity of
Arp 220. The luminosity surface density of the western nucleus ($ \log(\sigma
T^4) = 14.3\pm 0.2 ^{+0}_{-0.7}$ in units of L$_\odot$ kpc$^{-2}$) appears
sufficiently high to require the presence of an AGN or a "hot starburst",
although the exact value depends sensitively on the brightness distribution
adopted for the source. Although the role of any central AGN remains open, the
inferred mean gas column densities of $0.6-1.8 \times 10^{25}$ cm$^{-2}$ mean
that any AGN in Arp 220 must be Compton-thick.
