The Submillimeter Array 1.3 mm line survey of Arp 220

¹ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19 Chile
e-mail: smartin@eso.org
² Institut de Radioastronomie Milimétrique, 300 rue de la Piscine, 38406 Saint Martin d’Heres, France
³ Centro de Astrobiología (CSIC-INTA), Ctra. de Torrejón Ajalvir, km. 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁴ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Observatory, 439 92 Onsala, Sweden
⁵ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁶ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
⁷ Harvard-Smithsonian Center for Astrophysics, Submillimeter Array, 645 North A’ohoku Place, Hilo, HI 96720, USA
⁸ California Institute of Technology, Cahill Center for Astronomy and Astrophysics 301-17, Pasadena, CA 91125, USA
⁹ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen Ø, Denmark
¹⁰ Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, PR China

Received: 1 October 2010
Accepted: 9 December 2010

Abstract

Context. Though Arp 220 is the closest and by far the most studied ULIRG, a discussion is still ongoing on the main power source driving its huge infrared luminosity.

Aims. To study the molecular composition of Arp 220 in order to find chemical fingerprints associated with the main heating mechanisms within its nuclear region.

Methods. We present the first aperture synthesis unbiased spectral line survey toward an extragalactic object. The survey covered the 40 GHz frequency range between 202 and 242 GHz of the 1.3 mm atmospheric window.

Results. We find that 80% of the observed band shows molecular emission, with 73 features identified from 15 molecular species and 6 isotopologues. The ¹³C isotopic substitutions of HC₃N and transitions from HO, ²⁹SiO, and CH₂CO are detected for the first time outside the Galaxy. No hydrogen recombination lines have been detected in the 40 GHz window covered. The emission feature at the transition frequency of H31α line is identified to be an HC₃N molecular line, challenging the previous detections reported at this frequency. Within the broad observed band, we estimate that 28% of the total measured flux is due to the molecular line contribution, with CO only contributing 9% to the overall flux. We present maps of the CO emission at a resolution of 2.9′′ × 1.9′′ which, though not enough to resolve the two nuclei, recover all the single-dish flux. The 40 GHz spectral scan has been modelled assuming LTE conditions and abundances are derived for all identified species.

Conclusions. The chemical composition of Arp 220 shows no clear evidence of an AGN impact on the molecular emission but seems indicative of a purely starburst-heated ISM. The overabundance of H₂S and the low isotopic ratios observed suggest a chemically enriched environment by consecutive bursts of star formation, with an ongoing burst at an early evolutionary stage. The large abundance of water (~10^-5), derived from the isotopologue HO, as well as the vibrationally excited emission from HC₃N and CH₃CN are claimed to be evidence of massive star forming regions within Arp 220. Moreover, the observations put strong constraints on the compactness of the starburst event in Arp 220. We estimate that such emission would require  ~2−8 × 10⁶ hot cores, similar to those found in the Sgr B2 region in the Galactic center, concentrated within the central 700 pc of Arp 220.