2017A&A...601L...3P

We present the first detection of the ortho-H₂O 4₂₃-3₃₀ transition at 448GHz in space. We observed this transition in the local (z=0.010) luminous infrared (IR) galaxy ESO 320-G030 (IRAS F11506-3851) using the Atacama Large Millimeter/submillimeter Array (ALMA). The water 4₂₃-3₃₀ emission, which originates in the highly obscured nucleus of this galaxy, is spatially resolved over a region of ∼65pc in diameter and shows a regular rotation pattern compatible with the global molecular and ionized gas kinematics. The line profile is symmetric and well fitted by a Gaussian with an integrated flux of 37.0±0.7Jy.km/s. Models predict this water transition as a potential collisionally excited maser transition. On the contrary, in this galaxy, we find that the 4₂₃-3₃₀ emission is primarily excited by the intense far-IR radiation field present in its nucleus. According to our modeling, this transition is a probe of deeply buried galaxy nuclei thanks to the high dust optical depths (τ_100µm>1, N_H>10²⁴cm^–2) required to efficiently excite it.