Bright [C II] 158 μm emission in a quasar host galaxy at z = 6.54

The [C ii] 158 μm fine-structure line is known to trace regions of active star formation and is the main coolant of the cold, neutral atomic medium. In this Letter, we report a strong detection of the [C ii] line in the host galaxy of the brightest quasar known at $z>6.5$, the Pan-STARRS1 selected quasar PSO J036.5078+03.0498 (hereafter P036+03), using the IRAM NOEMA millimeter interferometer. Its [C ii] and total far-infrared luminosities are $(5.8\pm 0.7)\times {10}^9$ $L_{\odot }$ and $(7.6\pm 1.5)\times {10}^{12}$ $L_{\odot }$, respectively. This results in an $L_{\left[CII\right]}/L_{TIR}$ ratio of $\sim 0.8\times {10}^{-3}$, which is at the high end of those found for active galaxies, though it is lower than the average found in typical main-sequence galaxies at $z\sim 0$. We also report a tentative additional line that we identify as a blended emission from the $3_{22}-3_{13}$ and $5_{23}-4_{32}$ H₂O transitions. If confirmed, this would be the most distant detection of water emission to date. P036+03 rivals the current prototypical luminous J1148+5251 quasar at z = 6.42, in both rest-frame UV and [C ii] luminosities. Given its brightness and because it is visible from both hemispheres (unlike J1148+5251), P036+03 has the potential of becoming an important laboratory for the study of star formation and of the interstellar medium only ∼800 Myr after the Big Bang.