Molecular gas in z ∼ 6 quasar host galaxies

Abstract

We investigate the molecular gas content of z ∼ 6 quasar host galaxies using the Institut de Radioastronomie Millimétrique Northern Extended Millimeter Array. We targeted the 3 mm dust continuum, and the line emission from CO(6–5), CO(7–6), and [C I]₂₋₁ in ten infrared–luminous quasars that have been previously studied in their 1 mm dust continuum and [C II] 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 [C II] 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, [C I], or [C II] emission yields mass estimates of the entire sample in the range M_(H2) ∼ 10¹⁰–10¹¹ M_⊙. We compared the observed luminosities of dust, [C II], [C I], 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 NH ∼ 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.