Author: Clarissa Immisch

The [CII] 158μm emission line is one of the brightest cooling lines in star-forming galaxies and serves as a crucial tracer of star formation. Using (sub)millimeter telescopes like ALMA, strong [CII] emission has been detected in galaxies with z>5, providing valuable insights into early galaxy evolution.

Current cosmological simulations, such as IllustrisTNG (e.g. Nelson et al. 2019), can model galaxy formation and evolution from shortly after the Big Bang to the present day. These simulations enable us to test our ideas on the physics behind galaxy evolution, by comparing simulated galaxies with those observed in the real universe. However, the IllustrisTNG simulations lack direct information on [CII] emission, limiting our ability to compare simulated galaxies with ALMA observations.

To bridge this gap, we developed a subgrid model for [CII] emission within the SILCC simulation framework (e.g. Rathjen et al. 2023), building upon previous work, which used turbulent box (TB) simulations (Masterthesis Buhlmann 2022). Our approach systematically scans the SILCC simulation using cubic regions ("sub-boxes") of varying sizes, identifying those with mean densities ( 2 x 10^-25 to 10^-22  g cm^-3 ) matching previous TB simulations. The [CII] luminosity of these sub-boxes was then calculated using the assumption that the gas is optically thin everywhere (Bisbas et al. 2022). This process was repeated for multiple snapshots of the simulation, which exhibit varying star formation rates (SFRs).

Our analysis revealed that sub-boxes from high-SFR snapshots consistently produce the highest [CII] luminosities, and importantly, all SILCC sub-boxes exhibited significantly higher [CII] emission compared to TB simulations, due to SILCC’s inclusion of stellar feedback mechanisms.

We applied these [CII] emission estimates to 75 IllustrisTNG50 halos at redshifts z~ 4 - 6  (Muñoz-Elgueta et al. 2024) with stellar masses and star formation rates comparable to galaxies in the ALPINE (ALMA Large Program to Investigate C+ at Early Times) survey. Their surface brightness (SB) profiles were calculated and directly compared to observations from the ALPINE survey (Ginolfi et al. 2020).

The results (see Figure 1) show significant improvement over the previous method: while the TB-based model (in blue) underestimated [CII] emission by a factor of ~ 100, our SILCC-based approach (in red) overestimates emission by factors of ~ 40 on central galactic scales and ~ 10 on inner circumgalactic medium scales. This overestimation likely stems from several limitations in the approach. First, previous work (e.g. Franeck 2018) suggests that the [CII] line becomes optically thick at high densities, which leads to an overestimated emission in dense regions. Second, the SILCC simulation uses a simplified modeling for radiation fields, which may result in overestimated C  abundances by neglecting processes that would ionize carbon beyond C⁺.

Future work will implement the subgrid model in the SILCC-FUV simulation framework (Rathjen et al. 2025), which includes spatially and teporally varying FUV radiation fields, account for optical thickness effects at high densities and consider the varying levels of SF in the simulated halos. This work represents a significant step toward realistic [CII] emission predictions in cosmological simulations, essential for interpreting current ALMA observations and future facilities like the Fred Young Submillimeter Telescope.