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

¹ INAF, Osservatorio di Astrofisica e Scienza dello Spazio, Via P. Gobetti 93/3, I-40129 Bologna, Italy
² Università di Pisa, Dipartimento di Fisica “Enrico Fermi”, Largo Bruno Pontecorvo 3, Pisa I-56127, Italy
³ Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy
⁴ Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA
⁵ Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
⁶ Departamento de Astronomía, Universidad de Concepción, Barrio Universitario, Concepción, Chile
⁷ Caltech/IPAC, 1200 E. California Blvd., Pasadena, CA 91125, USA
⁸ University of Bologna, Department of Physics and Astronomy “Augusto Righi”, Via Gobetti 93/2, I-40129 Bologna, Italy
⁹ Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
¹⁰ Scuola Internazionale Superiore Studi Avanzati (SISSA), Physics Area, Via Bonomea 265, I-34136 Trieste, Italy
¹¹ Millenium Nucleus for Galaxies (MINGAL), Concepción, Chile

^⋆ Corresponding author: livia.vallini@inaf.it

Received: 16 April 2025
Accepted: 1 July 2025

Abstract

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 (t_dep); 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 t_dep in the EoR.

Methods. We selected galaxies at 4 < z < 9 from the SERRA 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 (κ_s), 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 κ_s; above this threshold, it also depends on κ_s, with burstier regions having lower conversion factors. This implies W_[CII] ∝ Σ_[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 Σ_gas in bright Σ_[CII] patches; this in turn flattens the KS slope and leads to overestimations of t_dep by up to a factor of 4.