Molecular gas in z ∼ 6 quasar host galaxies

¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, 40129 Bologna, Italy
e-mail: roberto.decarli@inaf.it
² Dipartimento di Fisica e Astronomia, Alma Mater Studiorum, Universitá di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
³ Dipartimento di Fisica “G. Occhialini”, Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
⁴ Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
⁶ National Radio Astronomy Observatory, Pete V. Domenici Array Science Center, PO Box O Socorro, NM 87801, USA
⁷ Argelander-Institute for Astronomy, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁸ Sorbonne Université, UPMC Université Paris 6 & CNRS, UMR 7095, Paris, France
⁹ Institut d’Astrophysique de Paris, 98b boulevard Arago, 75014 Paris, France
¹⁰ Steward Observatory, University of Arizona, 933 N. Cherry St., Tucson, AZ 85721, USA
¹¹ Gemini Observatory, NSF’s NOIRLab, 670 N A’ohoku Place, Hilo, Hawai’i 96720, USA
¹² Department of Physics, Winona State University, Winona, MN 55987, USA
¹³ ICRAR M468, The University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia
¹⁴ Research School of Astronomy and Astrophysics (RSAA), Australian National University, ACT 2611, Australia
¹⁵ Department of Astronomy, School of Physics, Peking University, 5 Yiheyuan Road, Haidian District, Beijing 10087, PR China
¹⁶ Kavli Institute of Astronomy and Astrophysics at Peking University, 5 Yiheyuan Road, Haidian District, Beijing 100871, PR China
¹⁷ Department of Astronomy, Tsinghua University, Beijing 100084, PR China
¹⁸ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Región Metropolitana, Chile
¹⁹ Institut de Radioastronomie Millimétrique (IRAM), 300 Rue de la Piscine, 38400 Saint-Martin-d’Hères, France
²⁰ Department of Astronomy, Cornell University, Space Sciences Building, Ithaca, NY 14853, USA
²¹ Astronomy Department, California Institute of Technology, MC249-17, Pasadena, California 91125, USA
²² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 9 December 2021
Accepted: 6 March 2022

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 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.