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Extended CII emission on tens of kiloparsecs, also known as a CII halo, is currently being reported around z ∼ 4 − 6 star-forming galaxies, especially thanks to the statistics of the ALPINE survey. The CII emission is expected to trace dense cold gas in the inner circumgalactic medium (CGM) of these galaxies. The origin of this emission is still debated. In this paper, we present a post-processing model applied to TNG50 star-forming galaxies at z ∼ 4 − 6, and we compare our results with the ALPINE observations. By incorporating C + abundances derived from UV background and young stars as radiation sources, we generated mock observations, from which we extracted surface-brightness (SB) profiles. We find that our model predicts similar CII emission values on galactic scales as the observations, providing validation for our approach. However, we find that the predicted CII emission in the inner CGM falls below the observed values by a factor of ∼10. We discuss several model limitations that may contribute to this discrepancy. We also find discrepancies with observations when comparing SB profiles of low and high star formation rate galaxies. Unlike the observations, simulations exhibit no discernible difference in the extended CII emission between the two subsamples. This discrepancy may reflect shortcomings in the feedback model of the simulation. Finally, our analysis suggests that the extended CII emission is likely a result of both gas from satellite galaxies and outflows from central galaxies, with satellites playing a dominant role within 0.6 < R / R vir < 1. A firm estimate of the importance of each contribution is beyond the scope of the current simulations.
Muñoz-Elgueta et al. (Thu,) studied this question.