Astronomy & Astrophysics

Free access article

A&A 468, L57-L61 (2007)
DOI: 10.1051/0004-6361:20077301

Letter

Black hole in the West nucleus of Arp 220

D. Downes¹ and A. Eckart^{2, 3}

¹ Institut de Radio Astronomie Millimétrique, Domaine Universitaire, 38406 St. Martin d'Hères, France
e-mail: downes@iram.fr
² I.Physikalisches Institut, Universität zu Köln, Zulpicherstrasse 77, 50937 Köln, Germany
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

(Received 14 February 2007 / Accepted 13 March 2007 )

Abstract
We present new observations with the IRAM Interferometer, in its longest-baseline configuration, of the CO(2-1) line and the 1.3 mm dust radiation from the Arp 220 nuclear region. The dust source in the West nucleus has a size of 0.19 $''\times 0.13''$ and a 1.3 mm brightness temperature of 90 K. This implies that the dust ring in the West nucleus has a high opacity, with $\tau = 1$ at 1.1 mm. 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'' 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 (10⁹ $M_\odot$ ), compact (radius 35 pc), and hot (true dust temperature 170 K). 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.

Key words: galaxies: nuclei -- galaxies: kinematics and dynamics -- galaxies: ISM -- galaxies: individual: Arp 220