First detection of the 448 GHz H2O transition in space

Pereira-Santaella, M. et. al.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 (τ100μm> 1, NH> 1024 cm-2) required to efficiently excite it. © ESO, 2017.M.P.S. acknowledges support from STFC through grant ST/N000919/1, the John Fell Oxford University Press (OUP) Research Fund and the University of Oxford. E.G.S., A.U., S.G.B., J.M.P., L.C., A.A.H., S.A., S.C., and F.R.V. acknowledge financial support by the Spanish MEC under grants ESP2015-65597-C4-1-R, AYA2012-32295, ESP2015-68694, AYA2013-42227-P and AYA2015-64346-C2-1-P, which is partly funded by the FEDER programme. E.G.A. a Research Associate at the Harvard-Smithsonian CfA and acknowledges support by NASA grant ADAP NNX15AE56G.Peer reviewe