Sub-arcsecond imaging of the water emission in Arp 220

TL;DR

This study uses high-resolution ALMA and VLA observations to analyze water emission in Arp 220, revealing that the emission likely originates from numerous star-forming regions rather than an active galactic nucleus.

Contribution

First detailed sub-arcsecond imaging of water emission in Arp 220 at multiple frequencies, clarifying its origin and excitation mechanisms.

Findings

Detected 183 GHz and 325 GHz water emission in both nuclei.

Emission is brighter in the western nucleus.

Water emission likely arises from star-forming regions, not an AGN.

Abstract

Extragalactic observations of water emission can provide valuable insights into the excitation of the interstellar medium. In addition, extragalactic megamasers are powerful probes of kinematics close to active nuclei. Therefore, it is paramount to determine the true origin of the water emission, whether it is excited by processes close to an AGN or in star-forming regions. We use ALMA Band 5 science verification observations to analyse the emission of the 183 GHz water line in Arp 220 on sub-arcsecond scales, in conjunction with new ALMA Band 7 data at 325 GHz. Specifically, the nature of the process leading to the excitation of emission at these water lines is studied in this context. Supplementary 22 GHz VLA observations are used to better constrain the parameter space in the excitation modelling of the water lines. We detect 183 GHz H2O 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 NH3 absorption line. 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.