2017A&A...602A..42K

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 183GHz H₂O 3₁₃-2₂₀ line, in conjunction with new ALMA Band 7 H₂O 5₁₅-4₂₂ data at 325GHz, and supplementary 22GHz H₂O 6₁₆-5₂₃ VLA observations, are used to better constrain the parameter space in the excitation modeling of the water lines.
Results. We detect 183GHz H₂O and 325GHz 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 325GHz lines show similar spectra and kinematics, but the 22GHz 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.