Widespread Extended [C II] Emission in High-redshift Galaxies: Insights from the FIRE-2 Cosmological Zoom-in Simulations
August 2026 • 2026ApJ..1007..123L
Abstract • Recent Atacama Large Millimeter/submillimeter Array observations reveal diffuse [C II] emission ("[C II] halos") extending to ∼10 kpc in galaxies at 4 < z < 6. These measurements provide new insights into high-redshift galactic ecosystems and processes that drive metal enrichment on inner circumgalactic scales. To better understand the nature of [C II] halos, we analyze a suite of high-redshift FIRE-2 simulations at 5 ≤ z ≤ 6 in the stellar mass range of 109─1010.5 M⊙. By postprocessing these simulations with three-dimensional dust radiative transfer and photoionization modeling, we generate synthetic images of [C II] and UV continuum emission, from which we extract one-dimensional surface brightness profiles. Our results reproduce both the galaxy-integrated and spatial distribution of [C II] and UV emission, capturing in particular the more extended profile of [C II] emission. Comparing the time evolution of [C II] halos with bursty star formation histories of the simulated galaxies, we find that [C II] emission becomes more spatially extended following the decline of star formation rate in recent starburst episodes. This implies a strong correlation between extended [C II] emission and bursty star formation, consistent with a key role for star formation-driven outflows in producing [C II] halos—though the kinematics of [C II]-emitting gas suggest that inflows and turbulent motions are also significant contributors. We also find a modest contribution from satellite galaxies to extended [C II] emission. Our framework can be readily applied to predict the observability of [C II] halos at higher redshifts and extended to create spatially resolved synthetic observations of other important emission lines, such as [O III] and Hα.
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