Left: Map of the 448 GHz (rest frequency) continuum (top panel) and zeroth moment of the H2O 423 − 330 emission (bottom panel) of ESO 320-G030. Right: Continuum subtracted profile of the H2O 423 − 330 448 GHz emission. See Pereira-Santaella et al. 2017 for further details.

Water is a key molecule to probe dense and warm regions in the interstellar medium, because of its chemistry as well as its coupling to the far-infrared. This work presents the first detection of the ortho-H2O 4_23 – 3_30 transition at 448 GHz in space. Due to a strong opacity in the terrestrial atmosphere, this line had not till now been observed in nearby molecular clouds. The line is found in emission with ALMA in a redshifted luminous infrared (IR) galaxy ESO 320-G030. Water excitation models have long predicted the maser nature of this transition, pumped by collisions when the kinetic temperature is around 1000K and the hydrogen density 10⁵ cm^-3. On the contrary, and surprisingly, the line is primarily excited by the intense far-IR radiation field that is present in the nucleus of this galaxy. This field extends over a region of 65 pc in diameter and shows a regular rotation pattern compatible with the global molecular and ionized gas kinematics. This H2O transition is therefore a probe of deeply buried galactic nuclei thanks to the high dust optical depths (NH > 4 10²⁴ cm²) required to efficiently excite it.

M. Pereira-Santaella, E. Gonzalez-Alfonso, A. Usero, et al. A&A 601, L3. DOI: https://doi.org/10.1051/0004-6361/201730851

with participation of J. Martín-Pintado, L. Colina, A. Alonso-Herrero, S. Arribas and F. Rico from CAB