2022ApJ...941...74O

We present our new Atacama Large Millimeter/submillimeter Array (ALMA) observations targeting CO(6-5) emission from three luminous Lyman-break galaxies (LBGs) at z_spec = 6.0293-6.2037 found in the Subaru/Hyper Suprime-Cam survey, whose [O III] 88 μm and [C II] 158 μm emissions have been detected with ALMA. We find a marginal detection of the CO(6-5) line from one of our LBGs, J0235-0532, at the ≃4σ significance level and obtain upper limits for the other two LBGs, J1211-0118 and J0217-0208. Our z = 6 luminous LBGs are consistent with the previously found correlation between the CO luminosity and the infrared luminosity. The unique ensemble of the multiple far-infrared emission lines and underlying continuum fed to a photodissociation region model reveals that J0235-0532 has a relatively high density of hydrogen nuclei n_H that is comparable to those of low-z (U)LIRGs, quasars, and Galactic star-forming regions with high n_H values, while the other two LBGs have lower n_H consistent with local star-forming galaxies. By carefully taking account of various uncertainties, we obtain constraints on total gas mass and gas surface density from their CO luminosity measurements. We find that J0235-0532 is located below the Kennicutt-Schmidt (KS) relation, comparable to the z = 5.7 LBG, HZ10, previously detected with CO(2-1). Combined with previous results for dusty starbursts at similar redshifts, the KS relation at z = 5-6 is on average consistent with the local one.