First CO(17-16) emission line detected in a z > 6 quasar
  We report the serendipitous detection of the CO(17-16) emission line toward
the quasar SDSSJ114816.64+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% of the total flux observed.
Photo-Dissociation 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.
