===== On OH+ in Extragalactic Sources =====

The first firm detection of of oxidaniumylidene or hydroxylium, OH<sup>+</sup>, was reported by\\
P. P. van der Werf, K. G. Isaak, R. Meijerink, et al.,\\
**[[https://doi.org/10.1051/0004-6361/201014682|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, no rest frequencies are given and no references for these. An abundance has been derived, but no details are given. Only a broad overview spectrum is shown. Thus, one may be inclined to view this detection with some caution. On the other hand, all three fine structure components of the //N// = 1 – 0 transition seem to have been observed in emission toward the ultraluminous infrared galaxy Mrk 231with correct relative intensities employing the SPIRE instrument on board of the //Herschel// satellite. Given the ubiquity and high abundance of OH<sup>+</sup> in Galactic sources, the identification is probably correct.

There have been prior reports on the tentative detection of OH<sup>+</sup>:\\
E. González-Alfonso, H, A. Smith, J. Fischer, and J. Cernicharo,\\
**[[https://doi.org/10.1086/422868|The Far-Infrared Spectrum of Arp 220]]**\\
//Astrophys. J.// **613**, (2004) 247–261\\
discuss the possibility that absorption features of NH observed with //ISO// LWS at 100 and 150 μm may be affected by the //N// = 3 – 2 and 2 – 1 transitions, respectively, of OH<sup>+</sup>. However, they refrain from suggesting that there is strong evidence for OH<sup>+</sup> to be present.\\
E. González-Alfonso, H. A. Smith, M. L. N. Ashby, J. Fischer, L. Spinoglio, and T. W. Grundy,\\
**[[https://doi.org/10.1086/527292|High-excitation OH and H<sub>2</sub>O Lines in Markarian 231: The Molecular Signatures of Compact Far-infrared Continuum Sources]]**\\
//Astrophys. J.// **675**, (2008) 303–315\\
assigned two barely detected (about 2.5 σ) absorption features detected near 150 μm with //ISO// LWS to the two strongest fine structure components of the //N// = 2 – 1 transition.\\
Detailed modeling may provide clues as to how plausible these observations are.

E. González-Alfonso, J. Fischer, S. Bruderer, et al.\\
reported on\\
**[[https://doi.org/10.1051/0004-6361/201220466|Excited OH<sup>+</sup>, H<sub>2</sub>O<sup>+</sup>, and H<sub>3</sub>O<sup>+</sup> in NGC 4418 and Arp 220]]**,\\
//Astron. Astrophys.// **550**, Art. No. A25 (2013);\\
using PACS on board of the //Herschel// satellite. Transitions up to //N// = 4 – 3 with upper state energy of almost 500 K were detected in both sources. There may be even evidence for the 5 – 4 components (//E//<sub>up</sub> ≈ 700 K) in NGC 4418, but the intensities do not fit the model.


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Contributor(s): H. S. P. Müller; 10, 2010; 04, 2013.

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