2024A&A...689A.220T

We used low- to high-frequency ALMA observations to investigate the cold gas and dust in ten quasistellar objects (QSOs) at z ≳ 6. Our analysis of the CO(6-5) and CO(7-6) emission lines in the selected QSOs provided insights into their molecular gas masses, which average around 10¹⁰ M_⊙. This is consistent with typical values for high-redshift QSOs. Proprietary and archival ALMA observations in bands 8 and 9 enabled precise constraints on the dust properties and star formation rate (SFR) of four QSOs in our sample for the first time. The examination of the redshift distribution of dust temperatures revealed a general trend of increasing T_dust with redshift, which agrees with theoretical expectations. In contrast, our investigation of the dust emissivity index indicated a generally constant value with redshift, suggesting shared dust properties among sources. We computed a mean cold dust spectral energy distribution considering all ten QSOs that offers a comprehensive view of the dust properties of high-z QSOs. The QSOs marked by a more intense growth of the supermassive black hole (HYPERION QSOs) showed lower dust masses and higher gas-to-dust ratios on average, but their H₂ gas reservoirs are consistent with those of other QSOs at the same redshift. The observed high SFR in our sample yields high star formation efficiencies and thus very short gas depletion timescales (τ_dep ∼ 10^–2 Gyr). Beyond supporting the paradigm that high-z QSOs reside in highly star-forming galaxies, our findings portrayed an interesting evolutionary path at z > 6. Our study suggests that QSOs at z ≳ 6 are undergoing rapid galaxy growth that might be regulated by strong outflows. In the M_BH - M_dyn plane, our high-z QSOs lie above the relation measured locally. Their inferred evolutionary path shows a convergence toward the massive end of the local relation, which supports the idea that they are candidate progenitors of local massive galaxies. The observed pathway involves intense black hole growth followed by substantial galaxy growth, in contrast with a symbiotic growth scenario. The evidence of a stellar bulge in one of the QSOs of the sample is further aligned with that typical of local massive galaxies.