The cold molecular gas in z ≳ 6 quasar host galaxies

Context. Probing the molecular gas reservoirs of z ≳ 6 quasar (QSO) host galaxies is fundamental to understanding the coevolution of star formation and black hole growth in these extreme systems. Yet, there is still an inhomogeneous coverage of molecular gas tracers for z ≳ 6 QSO hosts.
Aims: To measure the average excitation and mass of the molecular gas reservoirs in the brightest z > 6.5 QSO hosts, we combined new observations of CO(2-1) emission with existing observations of CO(6-5), CO(7-6), [C I] (2-1), [C II] 158 μm, and dust-continuum emission.
Methods: We reduced and analysed observations of CO(2-1), taken with the Karl G. Jansky Very Large Array, in three z = 6.5 − 6.9 QSO hosts - the highest redshift observations of CO(2-1) to date. By combining these with the nine z = 5.7 − 6.4 QSO hosts for which CO(2-1) emission has already been observed, we studied the spread in molecular gas masses and CO excitation of z ≳ 6 QSOs.
Results: Two of our three QSOs, P036+03 and J0305-3150, were not detected in CO(2-1), implying more highly excited CO than in the well-studied z = 6.4 QSO J1148+5251. However, we detected CO(2-1) emission at 5.1σ for our highest-redshift target, J2348-3054, yielding a molecular gas mass of (1.2 ± 0.2)×10¹⁰ M_⊙, assuming α_CO = 0.8 (K km s⁻¹ pc²)⁻¹ and r_{2, 1} = 1. This molecular gas mass is equivalent to the lower limit on the dynamical mass measured previously from resolved [C II] 158 μm observations, implying that there is little mass in stars or neutral gas within the [C II]-emitting region and that a low CO-to-H₂ conversion factor is applicable. On average, these z ≳ 6 QSO hosts have far higher CO(6-5)-, CO(7-6)-, and [C II] 158 μm versus CO(2-1) line ratios than the local gas-rich and IR-luminous galaxies that host active galactic nuclei, but with a large range of values, implying some variation in their interstellar medium conditions. We derived a mean CO(6-5)-to-CO(1-0) line luminosity ratio of r_{6, 1} = 0.9 ± 0.2.
Conclusions: Our new CO(2-1) observations show that even at 780 Myr after the Big Bang, QSO host galaxies can already have molecular gas masses of 10¹⁰ M_⊙, consistent with a picture in which these z ≳ 6 QSOs reside in massive starbursts that are coevolving with the accreting supermassive black holes. Their high gas versus dynamical masses and extremely high line excitation imply the presence of extremely dense and warm molecular gas reservoirs illuminated by strong interstellar radiation fields.