High-resolution HNC 3–2 SMA observations of Arp 220^*

¹ Department of Radio and Space Science with Onsala Observatory, Chalmers University of Technology, 439 94 Onsala, Sweden e-mail: saalto@chalmers.se
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
³ Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
⁴ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁵ Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141, Taipei 10617, Taiwan
⁶ National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan

Received: 16 June 2008
Accepted: 16 October 2008

Abstract

Aims. We study the properties of the nuclear molecular gas of the ultra luminous merger Arp 220 and effects of the nuclear source on gas excitation and chemistry. Specifically, our aim is to investigate the spatial location of the luminous HNC 3–2 line emission and address the underlying cause of its unusual brightness.

Methods. We present high resolution observations of HNC –2 with the submillimeter array (SMA).

Results. We find luminous HNC 3–2 line emission in the western part of Arp 220, centred on the western nucleus, while the eastern side of the merger shows relatively faint emission. A bright (36 K at resolution), narrow (60 ) emission feature emerges from the western nucleus, superposed on a broader spectral component. A possible explanation is weak maser emission through line-of-sight amplification of the background continuum source. There is also a more extended HNC 3–2 emission feature north and south of the nucleus. This feature resembles the bipolar OH maser morphology around the western nucleus. Substantial HNC abundances are required to explain the bright line emission from this warm environment – even when the high gas column density towards the western nucleus is taken into account. We discuss this briefly in the context of an X-ray affected chemistry and radiative excitation.

Conclusions. The luminous and possibly amplified HNC emission of the western nucleus of the Arp 220 merger reflects the unusual, and perhaps transient environment of the starburst/AGN activity there. The faint HNC line emission towards Arp 220-east reveals a real difference in physical conditions between the two merger nuclei.