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Widespread Extended [CII] Emission in High-Redshift Galaxies: Insights from the FIRE-2 Cosmological Zoom-in Simulations

Recent ALMA observations reveal diffuse [CII] emission (``[CII] halos") extending to $\sim 10\,$kpc in galaxies at $4 < z < 6$. These measurements provide new insights into high-redshift galactic ecosystems and processes that drive metal enrichment on circumgalactic scales. To better understand the nature of [CII] halos, we analyze a suite of high-redshift FIRE-2 simulations at $5 \leq z \leq 6$ in the stellar mass range of $10^{9}$--$10^{10.5}\,M_{\odot}$. By post-processing these simulations with three-dimensional dust radiative transfer and photoionization modeling, we generate synthetic images of [CII] and UV continuum emission, from which we extract one-dimensional surface brightness profiles. Our results reproduce both the galaxy-integrated and spatial distribution of [CII] and UV emission, capturing in particular the more extended profile of [CII] emission. Comparing the time evolution of [CII] halos with bursty star formation histories of the simulated galaxies, we find that [CII] emission becomes more spatially extended following the decline of star formation rate in recent starburst episodes. This implies a strong correlation between extended [CII] emission and bursty star formation, consistent with a key role for star formation-driven outflows in producing [CII] halos -- though the kinematics of [CII]-emitting gas suggest that inflows and turbulent motions are also significant contributors. We also find a modest contribution from satellite galaxies to extended [CII] emission. Our framework can be readily applied to predict the observability of [CII] halos at higher redshifts and extended to create spatially resolved synthetic observations of other important emission lines, such as [OIII] and H$α$.