2022ApJ...925..174V

We report CO(5 - 4) and CO(6 - 5) line observations in the dusty starbursting galaxy CRLE (z = 5.667) and the main-sequence (MS) galaxy HZ10 (z = 5.654) with the Northern Extended Millimeter Array. CRLE is the most luminous z > 5 starburst in the COSMOS field and HZ10 is the most gas-rich "normal" galaxy currently known at z > 5. We find line luminosities for CO(5 - 4) and CO(6 - 5) of (4.9 ± 0.5) and (3.8 ± 0.4) x 10¹⁰ K km s^–1 pc² for CRLE and upper limits of < 0.76 and < 0.60 x 10¹⁰ K km s^–1 pc² for HZ10, respectively. The CO excitation of CRLE appears comparable to other z > 5 dusty star-forming galaxies. For HZ10, these line luminosity limits provide the first significant constraints of this kind for an MS galaxy at z > 5. We find the upper limit of L_{5→4}^{′ }/L_{2→1}^{′ } in HZ10 could be similar to the average value for MS galaxies around z ≃ 1.5, suggesting that MS galaxies with comparable gas excitation may already have existed one billion years after the Big Bang. For CRLE we determine the most likely values for the H₂ density, kinetic temperature, and dust temperature based on excitation modeling of the CO line ladder. We also derive a total gas mass of (7.1 ± 1.3) x 10¹⁰M_☉. Our findings provide some of the currently most detailed constraints on the gas excitation that sets the conditions for star formation in a galaxy protocluster environment at z > 5.