2024ApJ...962..119L

We present observations using the Northern Extended Millimetre Array (NOEMA) of CO and H₂O emission lines and the underlying dust continuum in two quasars at z ∼ 6, i.e., P215-16 at z = 5.78 and J1429+5447 at z = 6.18. Notably, among all published CO spectral line energy distributions (SLEDs) of quasars at z ∼ 6, the two systems reveal the highest and the lowest CO levels of excitation, respectively. Our radiative transfer modeling of the CO SLED of P215-16 suggests that the molecular gas heated by active galactic nuclei could be a plausible origin for the high CO excitation. For J1429+5447, we obtain the first well-sampled CO SLED (from transitions from 2-1 to 10-9) of a radio-loud quasar at z ≳ 6. Analysis of the CO SLED suggests that a component from a single photodissociation region could explain the CO excitation in the radio-loud quasar J1429+5447. This work highlights the utility of the CO SLED in uncovering the properties of the interstellar medium in these young quasar-starburst systems at the highest redshift. The diversity of the CO SLEDs reveals the complexities in gas conditions and excitation mechanisms at their early evolutionary stage.