Cosmic evolution of the [C II]-to-dense neutral gas relation
Abstract
The [C II] 158 µm line is widely used to trace star formation and the gas contents of high-redshift galaxies. However, it remains unclear under which physical conditions it reliably traces the molecular reservoir, and whether a unique conversion factor α [C II] can be applied across cosmic time. We investigated the evolution of the relation between the [C II] luminosity and dense neutral gas mass from z ≃ 10 to z ≃ 0.2 using the VINTERGATAN simulation, a high-resolution cosmological zoom-in of a Milky Way-like galaxy. We post-processed the snapshots with the SKIRT radiative transfer code to generate synthetic [C II] data cubes. We measured global and spatially resolved (100 pc) relations between [C II] luminosity (L [C II] ), star formation rate (SFR), and dense neutral gas mass (M DN ). We followed the redshift evolution of the [C II]-to-dense neutral gas conversion factor α [C II] , and linked these trends to the evolution of the interstellar medium (ISM) phases. The global L [C II] -M DN and L [C II] -SFR relations evolve from a steep, [C II]-deficient regime at a very low metallicity to an almost linear behaviour, similar to calibrations at z ≈ 2, once the ISM reaches Z gas ≳ 0.05-0.1 Z ⊙ at z ≲ 5. Over this evolution, α [C II] spans nearly three orders of magnitude, from ≳10 4 down to ≈10 M ⊙ L -1 ⊙ , even though the [C II] emission remains spatially correlated with the dense neutral gas. We provide prescriptions for α [C II] based on integrated and local quantities. The latter recovers the dense neutral mass of VINTERGATAN within factors of the of order unity, whereas global relations depending only on L [C II] and/or the integrated metallicity fail by up to an order of magnitude during major and minor merger epochs. A unique, redshift-independent α [C II] therefore cannot recover dense neutral gas masses across the regimes we explored. [C II] remains a viable tracer of dense neutral gas at very high redshifts, but only when used with conversion factors that explicitly account for metallicity, ISM phase mix, and merger events.
Related
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- CHANG-ES. XLII. Cosmic-Ray Electron Transport and the Spatially Resolved Radio–SFR Relation in Edge-on Galaxies
BibTeX
@article{accard2026cosmic,
title = {{Cosmic evolution of the [C II]-to-dense neutral gas relation}},
author = {C. Accard and F. Renaud and K. Kraljic and D. Ismail and M. Béthermin and O. Agertz},
journal = {Astronomy \& Astrophysics - A\&A},
year = {2026},
month = aug,
doi = {10.1051/0004-6361/202660531},
url = {https://hal.science/hal-05853353v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Cosmic evolution of the [C II]-to-dense neutral gas relation
AU - C. Accard
AU - F. Renaud
AU - K. Kraljic
AU - D. Ismail
AU - M. Béthermin
AU - O. Agertz
PY - 2026
DA - 2026/08/24
JO - Astronomy & Astrophysics - A&A
DO - 10.1051/0004-6361/202660531
UR - https://hal.science/hal-05853353v1
AB - The [C II] 158 µm line is widely used to trace star formation and the gas contents of high-redshift galaxies. However, it remains unclear under which physical conditions it reliably traces the molecular reservoir, and whether a unique conversion factor α [C II] can be applied across cosmic time. We investigated the evolution of the relation between the [C II] luminosity and dense neutral gas mass from z ≃ 10 to z ≃ 0.2 using the VINTERGATAN simulation, a high-resolution cosmological zoom-in of a Milky Way-like galaxy. We post-processed the snapshots with the SKIRT radiative transfer code to generate synthetic [C II] data cubes. We measured global and spatially resolved (100 pc) relations between [C II] luminosity (L [C II] ), star formation rate (SFR), and dense neutral gas mass (M DN ). We followed the redshift evolution of the [C II]-to-dense neutral gas conversion factor α [C II] , and linked these trends to the evolution of the interstellar medium (ISM) phases. The global L [C II] -M DN and L [C II] -SFR relations evolve from a steep, [C II]-deficient regime at a very low metallicity to an almost linear behaviour, similar to calibrations at z ≈ 2, once the ISM reaches Z gas ≳ 0.05-0.1 Z ⊙ at z ≲ 5. Over this evolution, α [C II] spans nearly three orders of magnitude, from ≳10 4 down to ≈10 M ⊙ L -1 ⊙ , even though the [C II] emission remains spatially correlated with the dense neutral gas. We provide prescriptions for α [C II] based on integrated and local quantities. The latter recovers the dense neutral mass of VINTERGATAN within factors of the of order unity, whereas global relations depending only on L [C II] and/or the integrated metallicity fail by up to an order of magnitude during major and minor merger epochs. A unique, redshift-independent α [C II] therefore cannot recover dense neutral gas masses across the regimes we explored. [C II] remains a viable tracer of dense neutral gas at very high redshifts, but only when used with conversion factors that explicitly account for metallicity, ISM phase mix, and merger events.
SN - 0004-6361
SN - 1432-0746
ER -