First detection of the 448 GHz H2O transition in space

M. Pereira-Santaella, E. González-Alfonso, A. Usero, S. García-Burillo, J. Martín-Pintado, L. Colina, A. Alonso-Herrero, S. Arribas, S. Cazzoli, F. Rico, D. Rigopoulou, T. Storchi Bergmann. 2017. First detection of the 448 GHz H2O transition in space. Astronomy and Astrophysics 601, DOI: 10.1051/0004-6361/201730851

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 423 330 emission, which originates in the highly obscured nucleus of this galaxy, is spatially resolved over a region of similar to 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, NH > 10(24) cm(-2)) required to efficiently excite it.

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