Cosmic evolution of the [CII]-to-dense neutral gas relation

The [CII] 158 μ m 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[CII] can be applied across cosmic time. We investigated the evolution of the relation between the [CII] 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 [CII] data cubes. We measured global and spatially resolved (100 pc) relations between [CII] luminosity (L[CII]), star formation rate (SFR), and dense neutral gas mass (M DN). We followed the redshift evolution of the [CII]-to-molecular gas conversion factor α[CII], and link these trends to the evolution of the interstellar medium (ISM) phases. The global L[CII] - M DN and L[CII] -SFR relations evolve from a steep, [CII]-deficient regime at very low metallicity to an almost linear behaviour, similar to calibrations at z about2, once the ISM reaches Z 0.05 - 0.1,Z at z 5. Over this evolution, α[CII] spans nearly three orders of magnitude, from 10 4 down to about10,, even though the [CII] emission remains spatially correlated with the dense neutral gas. A unique, redshift-independent α[CII] therefore cannot recover dense neutral gas masses across the regimes we explored. [CII] 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.