We report the serendipitous detection of the CO(17–16) emission line towards the quasar sloan digital sky survey J114816.64+525150.3 (J1148) at redshift z ≃ 6.4 obtained with the Plateau de Bure Interferometer. The CO(17–16) line is possibly contaminated by OH+ emission, that may account for ∼35–60 per cent of the total flux observed. Photodissociation and X-ray-dominated regions (PDRs and XDRs) models show that PDRs alone cannot reproduce the high luminosity of the CO(17–16) line relative to low-J CO transitions and that XDRs are required. By adopting a composite PDR+XDR model, we derive molecular cloud and radiation field properties in the nuclear region of J1148. Our results show that highly excited CO lines represent a sensitive and possibly unique tool to infer the presence of X-ray faint or obscured supermassive black hole progenitors in high-z galaxies.
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Gallerani et al. (2014) studied this question.
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