Subarcsecond imaging of the water emission in Arp 220^⋆,⋆⋆

¹ Chalmers University of TechnologyDepartment of Earth and Space Sciences, Onsala Space Observatory, 43992, Onsala, Sweden
e-mail: sabine.koenig@chalmers.se
² European Southern Observatory (ESO), Alonso de Córdova 3107, Vitacura, Casilla 19001, 763 0355 Santiago, Chile
³ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, 763 0355 Santiago, Chile
⁴ Grupo de Astrofísica Molecular, Instituto de CC. de Materiales de Madrid (ICMM-CSIC), Sor Juana Inés de la Cruz 3, Cantoblanco, 28049 Madrid, Spain
⁵ Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141, 10617 Taipei, Taiwan
⁶ Max-Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁷ European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁸ Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, Domaine Universitaire, 38406 Saint-Martin-d’Hères, France
⁹ Laboratoire AIM, CEA/IRFU/Service d’Astrophysique, Bât. 709, 91191 Gif-sur-Yvette, France
¹⁰ National Radio Astronomy Observatory (NRAO), PO Box O, 1003 Lopezville Road, Socorro, NM 87801, USA
¹¹ New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA
¹² Observatorio Astronómico Nacional (OAN, IGN), Apdo 112, 28803 Alcalá de Henares, Spain
¹³ Observatorio de Madrid, OAN-IGN, Alfonso XII, 3, 28014 Madrid, Spain

Received: 22 December 2016
Accepted: 6 March 2017

Abstract

Aims. Extragalactic observations of water emission can provide valuable insight into the excitation of the interstellar medium. In particular they allow us to investigate the excitation mechanisms in obscured nuclei, that is, whether an active galactic nucleus or a starburst dominates.

Methods. We use subarcsecond resolution observations to tackle the nature of the water emission in Arp 220. ALMA Band 5 science verification observations of the 183 GHz H₂O 3₁₃ − 2₂₀ line, in conjunction with new ALMA Band 7 H₂O 5₁₅ − 4₂₂ data at 325 GHz, and supplementary 22 GHz H₂O 6₁₆ − 5₂₃ VLA observations, are used to better constrain the parameter space in the excitation modeling of the water lines.

Results. We detect 183 GHz H₂O and 325 GHz water emission toward the two compact nuclei at the center of Arp 220, being brighter in Arp 220 West. The emission at these two frequencies is compared to previous single-dish data and does not show evidence of variability. The 183 and 325 GHz lines show similar spectra and kinematics, but the 22 GHz profile is significantly different in both nuclei due to a blend with an NH₃ absorption line.

Conclusions. Our findings suggest that the most likely scenario to cause the observed water emission in Arp 220 is a large number of independent masers originating from numerous star-forming regions.