Molecular gas in<i>z</i>∼ 6 quasar host galaxies

<jats:p>We investigate the molecular gas content of<jats:italic>z</jats:italic> ∼ 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 <jats:sc>I</jats:sc>]<jats:sub>2−1</jats:sub>in ten infrared–luminous quasars that have been previously studied in their 1 mm dust continuum and [C <jats:sc>II</jats:sc>] 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<jats:italic>z</jats:italic> ∼ 6 quasars. The 3 mm to 1 mm flux density ratios are consistent with a modified black body spectrum with a dust temperature<jats:italic>T</jats:italic><jats:sub>dust</jats:sub> ∼ 47 K and an optical depth<jats:italic>τ</jats:italic><jats:sub><jats:italic>ν</jats:italic></jats:sub> = 0.2 at the [C <jats:sc>II</jats:sc>] frequency. Our study provides us with four independent ways to estimate the molecular gas mass,<jats:italic>M</jats:italic><jats:sub>H2</jats:sub>, 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<jats:italic>α</jats:italic><jats:sub>CO</jats:sub>and<jats:italic>α</jats:italic><jats:sub>[CII]</jats:sub>, the gas-to-dust mass ratio<jats:italic>δ</jats:italic><jats:sub>g/d</jats:sub>, and the carbon abundance [C]/H<jats:sub>2</jats:sub>. Leveraging either on the dust, CO, [C <jats:sc>I</jats:sc>], or [C <jats:sc>II</jats:sc>] emission yields mass estimates of the entire sample in the range<jats:italic>M</jats:italic><jats:sub>H2</jats:sub> ∼ 10<jats:sup>10</jats:sup>–10<jats:sup>11</jats:sup><jats:italic>M</jats:italic><jats:sub>⊙</jats:sub>. We compared the observed luminosities of dust, [C <jats:sc>II</jats:sc>], [C <jats:sc>I</jats:sc>], 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 (<jats:italic>n</jats:italic><jats:sub>H</jats:sub> > 10<jats:sup>4</jats:sup>cm<jats:sup>−3</jats:sup>) clouds with a column density<jats:italic>N</jats:italic><jats:sub>H</jats:sub> ∼ 10<jats:sup>23</jats:sup>cm<jats:sup>−2</jats:sup>, exposed to a radiation field with an intensity of<jats:italic>G</jats:italic><jats:sub>0</jats:sub> ∼ 10<jats:sup>3</jats:sup>(in Habing units). Our analysis reiterates the presence of massive reservoirs of molecular gas fueling star formation and nuclear accretion in<jats:italic>z</jats:italic> ∼ 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.</jats:p>