The first substantiated detection of Extragalactic H2O+
involves the J = 1.5 0.5 fine structure
component of the 111 000 transition of
ortho-oxidaniumyl near 1115 GHz, which has been detected in
absorption with the HIFI instrument on board of the Herschel
satellite toward M 82:
A. Weiß, M. A. Requena-Torres, R. Güsten, et al.
HIFI Spectroscopy of Low-Level Water Transitions in M 82
Astron. Astrophys. 521, (2010) Art. No. L1.
As may be interpreted from the title, H2O+ was observed by accident because of the proximity of that fine structure component to the 111 000 transition of para-H2O. However, a detailed analysis of this feature has been presented.
It is worthwhile mentioning that only the weaker
J = 0.5 0.5 fine structure
component of the 111 000
transition was reportedly detected with SPIRE/Herschel in
emission toward the ultraluminous infrared galaxy
P. P. van der Werf, K. G. Isaak, R. Meijerink, et al.,
Black Hole Accretion and Star-Formation as Drivers of Gas Excitation and Chemistry in Markarian 231
Astron. Astrophys. 518, (2010) Art. No. L42.
The account is somewhat sketchy, the report should therefore be viewed with some caution. On the other hand, the title seems to indicate why the line, provided the identification is correct, is seen in emission, in contrast to all presently known Galactic observations and also in contrast to the Extragalactic detection mentioned above.
E. González-Alfons1 , J. Fischer, S. Bruderer, et al.
Excited OH+, H2O+, and H3O+ in NGC 4418 and Arp 220,
Astron. Astrophys. 550, Art. No. A25 (2013);
using PACS on board of the Herschel satellite. Transitions 404 313 and 322 211 with upper state energies of about 300 K were detected in both sources with no or negligible overlap. Additional transitions with Eup beyond 400 K have also been observed, but are at least partially overlapped.