CO(7-6) and [C I](2-1) survey in z > 6 quasars
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| Format: | Preprint |
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2026
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| _version_ | 1866913147502198784 |
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| author | Xu, Fuxiang Decarli, Roberto Wang, Ran Borea, Anna Elisabetta Pensabene, Antonio Fan, Xiaohui Riechers, Dominik Bañados, Eduardo Weiß, Axel Costa, Michele Walter, Fabian Wang, Feige Yang, Jinyi Venemans, Bram Li, Jianan Farina, Emanuele Paolo |
| author_facet | Xu, Fuxiang Decarli, Roberto Wang, Ran Borea, Anna Elisabetta Pensabene, Antonio Fan, Xiaohui Riechers, Dominik Bañados, Eduardo Weiß, Axel Costa, Michele Walter, Fabian Wang, Feige Yang, Jinyi Venemans, Bram Li, Jianan Farina, Emanuele Paolo |
| contents | High-redshift ($z\gtrsim6$) quasars trace the earliest supermassive black holes and intense star formation, offering key laboratories for black hole-galaxy evolution at cosmic dawn. While far-infrared studies have revealed large dust reservoirs and strong [C II] emission, the physical conditions and molecular gas content of their ISM remain uncertain. We present ALMA Band 3 observations of the redshifted CO(7-6) and [C I](2-1) emission lines and dust continuum in a sample of 18 quasars at $z \sim 6$. We detected CO(7-6) in 15/18, [C I](2-1) in 6/18, and continuum in 13/18 sources. Line luminosities and continuum fluxes were used to estimate molecular gas masses from CO, [C I], and dust, and a hierarchical Bayesian cross-calibration of all four tracers yielded consistent per-source $M_{\rm H_2}$ estimates and conversion factors. Comparison with PDR and XDR model grids using the $L_{\rm [CII]}/L_{\rm [CI]}$ and $L_{\rm CO(7--6)}/L_{\rm TIR}$ ratios suggests gas densities of $n > 10^4$ cm$^{-3}$ and radiation fields of $G_0 \sim 10^3$--$10^4$ for sources consistent with PDR solutions, while many quasars fall outside the model parameter space. The $L'_{\rm CO(7-6)}/L'_{\rm [CI](2-1)}$ ratio indicates that a large fraction of the molecular gas resides in a warm and highly excited phase. Together these results suggest that classical PDR heating alone cannot explain the observed line ratios and that additional volumetric processes such as X-ray irradiation, turbulence and shocks, or enhanced cosmic-ray heating likely influence the excitation of the cold ISM. They demonstrate the power of multi-line diagnostics in revealing the excitation and structure of the cold ISM in early quasar host galaxies and highlight the need for joint analysis of CO, [C I], [C II], and dust emission to characterize star formation and AGN-driven heating at cosmic dawn. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2605_20698 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | CO(7-6) and [C I](2-1) survey in z > 6 quasars Xu, Fuxiang Decarli, Roberto Wang, Ran Borea, Anna Elisabetta Pensabene, Antonio Fan, Xiaohui Riechers, Dominik Bañados, Eduardo Weiß, Axel Costa, Michele Walter, Fabian Wang, Feige Yang, Jinyi Venemans, Bram Li, Jianan Farina, Emanuele Paolo Astrophysics of Galaxies High-redshift ($z\gtrsim6$) quasars trace the earliest supermassive black holes and intense star formation, offering key laboratories for black hole-galaxy evolution at cosmic dawn. While far-infrared studies have revealed large dust reservoirs and strong [C II] emission, the physical conditions and molecular gas content of their ISM remain uncertain. We present ALMA Band 3 observations of the redshifted CO(7-6) and [C I](2-1) emission lines and dust continuum in a sample of 18 quasars at $z \sim 6$. We detected CO(7-6) in 15/18, [C I](2-1) in 6/18, and continuum in 13/18 sources. Line luminosities and continuum fluxes were used to estimate molecular gas masses from CO, [C I], and dust, and a hierarchical Bayesian cross-calibration of all four tracers yielded consistent per-source $M_{\rm H_2}$ estimates and conversion factors. Comparison with PDR and XDR model grids using the $L_{\rm [CII]}/L_{\rm [CI]}$ and $L_{\rm CO(7--6)}/L_{\rm TIR}$ ratios suggests gas densities of $n > 10^4$ cm$^{-3}$ and radiation fields of $G_0 \sim 10^3$--$10^4$ for sources consistent with PDR solutions, while many quasars fall outside the model parameter space. The $L'_{\rm CO(7-6)}/L'_{\rm [CI](2-1)}$ ratio indicates that a large fraction of the molecular gas resides in a warm and highly excited phase. Together these results suggest that classical PDR heating alone cannot explain the observed line ratios and that additional volumetric processes such as X-ray irradiation, turbulence and shocks, or enhanced cosmic-ray heating likely influence the excitation of the cold ISM. They demonstrate the power of multi-line diagnostics in revealing the excitation and structure of the cold ISM in early quasar host galaxies and highlight the need for joint analysis of CO, [C I], [C II], and dust emission to characterize star formation and AGN-driven heating at cosmic dawn. |
| title | CO(7-6) and [C I](2-1) survey in z > 6 quasars |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2605.20698 |