2022A&A...662A..60D

We investigate the molecular gas content of z ∼ 6 quasar host galaxies using theInstitut de Radioastronomie Millimetrique Northern Extended Millimeter Array. We targeted the 3 mm dust continuum, and the line emission from CO(6-5), CO(7-6), and [CI]_2–1 in ten infrared-luminous quasars that have been previously studied in their 1 mm dust continuum and [CII] line emission. We detected CO(7-6) at various degrees of significance in all the targeted sources, thus doubling the number of such detections in z ∼ 6 quasars. The 3 mm to 1 mm flux density ratios are consistent with a modified black body spectrum with a dust temperature T_dust ∼ 47 K and an optical depth τ_ν = 0.2 at the [CII] frequency. Our study provides us with four independent ways to estimate the molecular gas mass, M_H2, in the targeted quasars. This allows us to set constraints on various parameters used in the derivation of molecular gas mass estimates, such as the mass per luminosity ratios α_CO and α_[CII], the gas-to-dust mass ratio δ_g/d, and the carbon abundance [C]/H₂. Leveraging either on the dust, CO, [CI], or [CII] emission yields mass estimates of the entire sample in the range M_H2 ∼ 10¹⁰-10¹¹ M_☉. We compared the observed luminosities of dust, [CII], [CI], and CO(7-6) with predictions from photo-dissociation and X-ray dominated regions. We find that the former provide better model fits to our data, assuming that the bulk of the emission arises from dense (n_H > 10⁴ cm^–3) clouds with a column density N_H ∼ 10²³ cm^–2, exposed to a radiation field with an intensity of G₀ ∼ 10³ (in Habing units). Our analysis reiterates the presence of massive reservoirs of molecular gas fueling star formation and nuclear accretion in z ∼ 6 quasar host galaxies.It also highlights the power of combined 3 mm and 1 mm observations for quantitative studies of the dense gas content in massive galaxies at cosmic dawn.