Widespread Extended [C II ] Emission in High-redshift Galaxies: Insights from the FIRE-2 Cosmological Zoom-in Simulations

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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 10 9 –10 10.5 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 H α .
Funding Information
  • National Radio Astronomy Observatory (SOSPADA-037)
  • National Science Foundation (AST-2108230)
  • National Science Foundation (AST-2307327)
  • National Aeronautics and Space Administration (21-ATP21-0036)
  • National Aeronautics and Space Administration (23-ATP23-0008)
  • Space Telescope Science Institute (JWST-AR-03252.001-A)
  • United States-Israel Binational Science Foundation (2024193)

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