Spatially resolved [CII]–gas conversion factor in early galaxies

Aims. Determining how efficiently gas collapses into stars at high redshifts is key to understanding galaxy evolution in the epoch of reionization (EoR). Globally, this process is quantified by the gas depletion time (tdep); on resolved scales, it is quantified by the slope and normalization of the Kennicutt-Schmidt (KS) relation. This work explores the global (α[CII]) and spatially resolved (W[CII]) [CII]-to-gas conversion factors at high-z and their use when inferring gas masses, surface densities, and tdep in the EoR. Methods. We selected galaxies at 4 < z < 9 from the <ns0:monospace>SERRA</ns0:monospace> cosmological zoom-in simulation, which features on-the-fly radiative transfer and resolves interstellar medium properties down to ≈30 pc. The [CII] emission modeling from photodissociation regions allows us to derive the global α[CII] and maps of W[CII]. We study their dependence on gas metallicity (Z), density (n), Mach number (ℳ), and burstiness parameter (κs), and provide best-fit relations. Results. The α[CII] decreases with increasing Z and galaxy compactness, while the resolved W[CII] shows two regimes: at Z < 0.2 Z⊙, it anticorrelates with n and Z but not with κs; above this threshold, it also depends on κs, with burstier regions having lower conversion factors. This implies $ W_{\mathrm{[CII]}}\propto \Sigma_{\mathrm{[CII]}}^{-0.5} $, as dense, metal-rich, and bursty regions exhibit higher [CII] surface brightnesses. Applying a constant α[CII] leads to an overestimation of Σgas in bright Σ[CII] patches; this in turn flattens the KS slope and leads to overestimations of tdep by up to a factor of 4.