Sub-arcsecond imaging of the water emission in Arp 220.

Aims: Extragalactic observations of water emission can provide valuable insights into the excitation of the interstellar medium. In particular they allow us to investigate the excitation mechanisms in obscured nuclei, i.e. whether an active galactic nucleus or a starburst dominate.

Methods: We use sub-arcsecond resolution observations to tackle the nature of the water emission in Arp 220. ALMA Band 5 science verification observations of the 183 GHz HO 3-2 line, in conjunction with new ALMA Band 7 HO 5-4 data at 325 GHz, and supplementary 22 GHz HO 6 - 5 VLA observations, are used to better constrain the parameter space in the excitation modelling of the water lines.

Results: We detect 183 GHz HO and 325 GHz water emission towards 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 NH 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.