Black hole in the West Nucleus of Arp 220

TL;DR

This paper presents high-resolution observations of the West nucleus of Arp 220, revealing a compact, hot, optically thick dust ring likely heated by an obscured active galactic nucleus, distinct from typical starburst sources.

Contribution

It provides detailed imaging and analysis of the dust and molecular gas structure in Arp 220's West nucleus, suggesting the presence of an obscured AGN as the heating source.

Findings

The dust ring is compact, massive, and hot with a radius of 35 pc.

The dust ring is optically thick and resembles quasar dust structures.

The heating source is likely an obscured AGN, not a starburst.

Abstract

We present new observations with the IRAM Interferometer, in its longest-baseline configuration, of the CO(2-1) line and the 1.3mm dust radiation from the Arp 220 nuclear region. The dust source in the West nucleus has a size of 0.19 x 0.13 arcsec and a 1.3mm brightness temperature of 90K. This implies that the dust ring in the West nucleus has a high opacity, with tau = 1 at 1.1mm. Not only is the dust ring itself optically thick in the submm and far-IR, but it is surrounded by the previously-known, rapidly rotating molecular disk of size 0.5 arcsec that is also optically thick in the mid-IR. The molecular ring is cooler than the hot dust disk because the CO(2-1) line is seen in absorption against the dust disk. The dust ring is massive (1E9 solar masses), compact (radius 35pc), and hot (true dust temperature 170K). It resembles rather strikingly the dust ring detected around the quasar APM 08279+52, and is most unlike the warm, extended dust sources in starburst galaxies. Because there is a strong temperature gradient from the hot dust ring to the cooler molecular disk, the heating must come from a concentrated source, an AGN accretion disk that is completely invisible at optical wavelengths, and heavily obscured in hard X-rays.