We show that perturbative techniques inspired by effective field theory (EFT) can be used to “paint on” the large-scale 21 cm field during reionization using only the underlying linear density field. It is, therefore, possible to enlarge or “supersize” hydrodynamical simulations at low resolution, on scales that are larger than the nonlinear scale of the 21 cm field. In particular, the EFT provides a mapping between the linear density field and the 21 cm field. We show that this mapping can be reliably extracted from relatively small simulation volumes using the thesan suite of simulations, which have a comoving volume of $(95.5\mathrm{Mpc}{)}^3$. Specifically, we show that, if we fit the EFT coefficients in a small $\sim 5\%$ subvolume of the simulation, we can predict the 21 cm field to within $\mathcal{O}(10\%)$ accuracy in the rest of the simulation given only the linear density field. We show that our technique is robust to different models of dark matter and differences in the subgrid reionization modeling.

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• Cosmology
• Large scale structure of the Universe
• Radio, microwave, & sub-mm astronomy
• Astrophysical & cosmological simulations