Molecular gas in z similar to 6 quasar host galaxies

We investigate the molecular gas content of z similar to 6 quasar host galaxies using the Institut de Radioastronomie Millimetrique Northern Extended Millimeter Array. We targeted the 3mm 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 1mm 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 similar to 6 quasars. The 3mm to 1mm flux density ratios are consistent with a modified black body spectrum with a dust temperature T-dust similar to 47K and an optical depth tau(nu) = 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 alpha(CO) and alpha([CII]), the gas-to-dust mass ratio delta(g/d), and the carbon abundance [C]/H-2. Leveraging either on the dust, CO, [C i], or [C II] emission yields mass estimates of the entire sample in the range M-H2 similar to 10(10)-10(11) M-circle dot. 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 (nH > 10(4) cm(-3)) clouds with a column density N-H similar to 10(23) cm(-2), exposed to a radiation field with an intensity of G(0) similar to 10(3) (in Habing units). Our analysis reiterates the presence of massive reservoirs of molecular gas fueling star formation and nuclear accretion in z similar to 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.