First detection of the 448 GHz H2O transition in space

Abstract

We present the first detection of the ortho-H2O 423����330 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 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 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 ( 100 m > 1, NH > 1024 cm����2) required to e ciently excite it.