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 \leq 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 .