Molecular gas inz∼ 6 quasar host galaxies

Abstract

We investigate the molecular gas content ofz ∼ 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]2−1in 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 inz ∼ 6 quasars. The 3 mm to 1 mm flux density ratios are consistent with a modified black body spectrum with a dust temperatureTdust ∼ 47 K and an optical depthτν = 0.2 at the [C II] frequency. Our study provides us with four independent ways to estimate the molecular gas mass,MH2, 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 ratiosαCOandα[CII], the gas-to-dust mass ratioδg/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 rangeMH2 ∼ 1010–1011M⊙. 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 > 104cm−3) clouds with a column densityNH ∼ 1023cm−2, exposed to a radiation field with an intensity ofG0 ∼ 103(in Habing units). Our analysis reiterates the presence of massive reservoirs of molecular gas fueling star formation and nuclear accretion inz ∼ 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.