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