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Published June 2022 | Published + Accepted Version
Journal Article Open

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.

Additional Information

© ESO 2022. Received: 9 December 2021 Accepted: 6 March 2022. We are grateful to the referee for their useful and thorough report that helped to improve the manuscript. This work is based on observations carried out under project numbers S052, X04D, S15DA, S17CD, S18DM, S19DM with the IRAM NOEMA Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). The research leading to these results has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 730562 [RadioNet]. R.D. is grateful to the IRAM staff and local contacts for their help and support in the data processing. F.W. and B.P.V. acknowledge support from ERC Advanced grant 740246 (Cosmic_Gas). D.R. acknowledges support from the National Science Foundation under grant numbers AST-1614213 and AST-1910107. D.R. also acknowledges support from the Alexander von Humboldt Foundation through a Humboldt Research Fellowship for Experienced Researchers. F.B. acknowledges support through the within the Collaborative Research Centre 956, sub-project A01, funded by the Deutsche Forschungsgemeinschaft (DFG) – project ID 184018867.

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Accepted Version - 2203.03658.pdf

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Additional details

Created:
August 22, 2023
Modified:
October 24, 2023