Molecular gas in z~6 quasar host galaxies

We investigate the molecular gas content of z~6 quasar host galaxies using the IRAM / Northern Extended Millimeter Array. We target the 3mm dust continuum, and the line emission from CO(6-5), CO(7-6), [CI]2-1 in 10 infra-red-luminous quasars that have been previously studied in their 1mm dust continuum and [CII] line emission. We detect 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 3mm to 1mm flux density ratios are consistent with a modified black body spectrum with a dust temperature \(T_{dust}\)~47 K and an optical depth \(\tau_{\nu}\)=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 \(\alpha_{CO}\) and \(\alpha_{[CII]}\), the gas-to-dust ratio \(\delta_{g/d}\), 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 \(M_{H2}\)~\(10^{10}\) to \(10^{11}\) M\(_{\odot}\). We compare 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 (\(n_H>10^4\) cm\(^{-3}\)) clouds with a column density \(N_{H}\)~\(10^{23}\) cm\(^{-2}\), exposed to a radiation field with intensity \(G_0\)~\(10^3\) (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 3mm and 1mm observations for quantitative studies of the dense gas content in massive galaxies at cosmic dawn.