Extreme dust disks in Arp 220 as revealed by ALMA

TL;DR

This study uses high-resolution ALMA imaging to analyze dust properties and luminosity in Arp 220's nuclei, revealing extreme dust opacity, high luminosity densities, and implications for hidden AGN activity in this ultraluminous infrared galaxy.

Contribution

First high-frequency, high-resolution ALMA images of Arp 220 revealing detailed dust and luminosity characteristics of its nuclei.

Findings

Western nucleus contains warm, optically thick dust at 200 K.

Luminosity surface density suggests possible AGN or hot starburst.

Nuclei account for over 80% of Arp 220's infrared luminosity.

Abstract

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.