# First detection of the 448 GHz H2O transition in space

## TL;DR

This paper reports the first space detection of the 448 GHz ortho-H2O transition in a galaxy, revealing insights into deeply buried galactic nuclei through high dust optical depth excitation.

## Contribution

It presents the first detection and spatially resolved observation of the 448 GHz H2O transition in space, demonstrating its excitation by IR radiation in a galaxy nucleus.

## Key findings

- Detection of the 448 GHz H2O line in ESO 320-G030
- Line is excited by IR radiation, not maser activity
- Transition probes deeply obscured galaxy nuclei

## Abstract

We present the first detection of the ortho-H2O 4_23-3_30 transition at 448 GHz 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 4_23-3_30 emission, which originates in the highly obscured nucleus of this galaxy, is spatially resolved over a region of ~65 pc 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 +- 0.7 Jy km s-1 . Models predict this water transition as a potential collisionally excited maser transition. On the contrary, in this galaxy, we find that the 4_23-3_30 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 (tau_100{\mu}m > 1, N_H > 1e24 cm-2) required to efficiently excite it.