We show that perturbative techniques inspired by effective field theory (EFT) can be used to "paint on" the 21~cm field during reionization using only the underlying linear density field. This procedure is accurate to within O(10%) on large scales and thus can be used to enlarge or "supersize" hydrodynamical simulations. In particular, the EFT provides a mapping between the linear density field and a nonlinear tracer field, both in real and redshift space. 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 Mpc)^3. Specifically, we show that if we fit the EFT coefficients in a small ~5% sub-volume of the simulation, we can accurately predict the 21cm field 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 sub-grid reionization modeling.
@article{arxiv.2504.02929,
title = {Supersizing hydrodynamical simulations of reionization using perturbative techniques},
author = {Wenzer Qin and Katelin Schutz and Olivia Rosenstein and Stephanie O'Neil and Mark Vogelsberger},
journal= {arXiv preprint arXiv:2504.02929},
year = {2025}
}