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