Constraining the Excitation of Molecular Gas in Two Quasar-starburst Systems at z ∼ 6

We present NOrthern Extended Millimeter Array observations of CO(8-7), (9-8), and (10-9) lines, as well as the underlying continuum for two far-infrared luminous quasars: SDSS J2054-0005 at z = 6.0389 and SDSS J0129-0035 at z = 5.7788. Both quasars were previously detected in CO (2-1) and (6-5) transitions, making them candidates for studying the CO spectral line energy distribution (SLED) of quasars at z ∼ 6. Utilizing the radiative transfer code CLOUDY, we fit the CO SLED with two heating mechanisms, including the photodissociation region (PDR) and X-ray-dominated region (XDR) for both objects. The CO SLEDs can be fitted by either a dense PDR component with an extremely strong far-ultraviolet radiation field (gas density n _H ∼ 10⁶ cm⁻³ and field strength G ₀ ≳ 10⁶) or a two-component model including a PDR and an XDR. However, the line ratios, including L _TIR and previous [C ii]_{158 μm} and [C i]_{369 μm} measurements, argue against a very high PDR radiation field strength. Thus, the results prefer a PDR+XDR origin for the CO SLED. The excitation of the high-J CO lines in both objects is likely dominated by the central active galactic nucleus (AGN). We then check the CO (9-8)-to-(6-5) line luminosity ratio r ₉₆ for all z ∼ 6 quasars with available CO SLEDs (seven in total) and find that there are no clear correlations between r ₉₆ and both L _FIR and the AGN UV luminosities. This further demonstrates the complexity of the CO excitation powered by both the AGN and nuclear star formation in these young quasar host galaxies.