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 × 0.″20) ever obtained for this prototypical ultraluminous infrared galaxy. The western nucleus is revealed to contain warm (200 K) dust that is optically thick (τ434 μm = 5.3), while the eastern nucleus is cooler (80 K) and somewhat less opaque (τ434 μm = 1.7). We derive full width at half-maximum diameters of 76 × ⩽ 70 pc and 123 × 79 pc for the western and eastern nucleus, respectively. The two nuclei combined account for ( (calibration) (systematic))% of the total infrared luminosity of Arp 220. The luminosity surface density of the western nucleus ( in units of L☉ kpc−2) appears sufficiently high to require the presence of an active galactic nucleus (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) × 1025 cm−2 mean that any AGN in Arp 220 must be Compton-thick.