First CO(17-16) emission line detected in a z > 6 quasar

TL;DR

This paper reports the first detection of the CO(17-16) emission line in a quasar at redshift 6.4, revealing the importance of X-ray dominated regions for understanding molecular excitation in early galaxies.

Contribution

It presents the first observation of high-J CO emission in a z > 6 quasar and demonstrates the necessity of XDRs to explain the line's luminosity, advancing knowledge of early galaxy nuclei.

Findings

Detection of CO(17-16) line in a z=6.4 quasar.

High-J CO lines indicate presence of X-ray dominated regions.

XDRs are essential to explain observed molecular line luminosities.

Abstract

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.