Observed [C II]/[C I] luminosity line ratio, as a function of the CO(7–6)/IR luminosity ratio (left panel), for the quasars in our study (filled circles; see references in the main text), as well as other high redshift (z > 1) galaxies and quasar hosts (filled squares) and local galaxies (empty stars, based on Rosenberg et al. 2015). All the quasars in our study show [C II]/[C I] ratios ≳20, which are not compatible with XDRs. Our sample also displays a range of CO(7–6)/IR ratios comparable with other high–z sources, but a factor ∼2−3× higher than the bulk of the local galaxies. Right: predictions of the [C II]/[C I] ratio as a function of the CO(7–6)/IR luminosity ratio for various gas densities log n_H[cm⁻³] = (3, 4, 5) for a PDR with incident radiation field log G₀ = (2, 3, 4) and an XDR with log F_X = ( − 1, 0, +1). Our sources (shown in filled circled, with the same color scheme as in the left-hand panel) are best described by a PDR–like environment with N_H ∼ 10²³ cm⁻², n_H ≳ 10⁴ cm⁻³, and G₀ ∼ 10³.