High-redshift (z emission, the physical condition and molecular gas content of their interstellar medium (ISM) remain uncertain. We present sensitive Atacama Large Millimeter/submillimeter Array Band 3 observations of the redshifted CO (7--6) and quasars are signposts of the earliest supermassive black holes and intense star formation, offering key laboratories for black hole--galaxy evolution at cosmic dawn. While far-infrared studies have revealed large dust reservoirs and strong C, ii C, i (2--1) emission lines and the underlying dust continuum in a sample of 18 quasars at z ∼ 6. We detected CO (7--6) in 15/18, C, i (2--1) in 6/18, and continuum in 13/18 sources. Line luminosities and continuum fluxes were used to estimate molecular gas masses from CO, C, i, and dust, and a hierarchical Bayesian cross-calibration of all four tracers yielded consistent per-source M_̊m H₂ estimates and global conversion factors. Comparison with photodissociation region (PDR) and X-ray dominated region model grids using the L_ ̊m CII /L_ ̊m CI and L_ ̊m CO (7 6) -- /L_ ̊m TIR ratios suggests gas densities of n > 10⁴ cm -3 and radiation fields of G₀ ∼ 10³–10⁴ for the subset of sources consistent with PDR solutions, while many quasars fall outside the model parameter space. Additional diagnostics based on the L'_ ̊m CO (7-6) /L'_ ̊m CI (2-1) ratio indicate that a large fraction of the molecular gas resides in a warm and highly excited phase. Together these results suggest that classical PDR heating alone cannot explain the observed line ratios and that additional volumetric processes such as X-ray irradiation, turbulence and shocks, or enhanced cosmic-ray heating likely influence the excitation of the cold ISM. These results demonstrate the power of multi-line diagnostics in revealing the excitation and structure of the cold ISM in early quasar host galaxies and highlight the need for a joint analysis of CO, C, i, C, ii, and dust emission to fully characterize star formation and heating driven by active galactic nuclei at cosmic dawn.
Xu et al. (Thu,) studied this question.