We introduce the thesan project, a suite of large volume ($L_\mathrm{box} = 95.5 \, \mathrm{cMpc}$) radiationmagnetohydrodynamic simulations that simultaneously model the largescale statistical properties of the intergalactic medium during reionization and the resolved characteristics of the galaxies responsible for it. The flagship simulation has dark matter and baryonic mass resolutions of $3.1 \times 10^6\, {\rm M_\odot }$ and $5.8 \times 10^5\, {\rm M_\odot }$, respectively. The gravitational forces are softened on scales of 2.2 ckpc with the smallest cell sizes reaching 10 pc at z = 5.5, enabling predictions down to the atomic cooling limit. The simulations use an efficient radiation hydrodynamics solver (areport) that precisely captures the interaction between ionizing photons and gas, coupled to welltested galaxy formation (IllustrisTNG) and dust models to accurately predict the properties of galaxies. Through a complementary set of medium resolution simulations we investigate the changes to reionization introduced by different assumptions for ionizing escape fractions, varying dark matter models, and numerical convergence. The fiducial simulation and model variations are calibrated to produce realistic reionization histories that match the observed evolution of the global neutral hydrogen fraction and electron scattering optical depth to reionization. They also match a wealth of highredshift observationally inferred data, including the stellartohalomass relation, galaxy stellar mass function, star formation rate density, and the mass–metallicity relation, despite the galaxy formation model being mainly calibrated at z = 0. We demonstrate that different reionization models give rise to varied bubble size distributions that imprint unique signatures on the 21 cm emission, especially on the slope of the power spectrum at large spatial scales, enabling current and upcoming 21 cm experiments to accurately characterize the sources that dominate the ionizing photon budget.
Efficient and accurate simulations of the reionization epoch are crucial to exploring the vast uncharted parameter space that will soon be constrained by measurements of the 21cm power spectrum. One of these parameters, Rmax, is meant to characterize the absorption of photons by residual neutral gas inside of ionized regions, but has historically been implemented in a very simplistic fashion acting only as a maximum distance that ionizing photons can travel. We leverage the correspondence between excursion set methods and the integrated flux from ionizing sources to define two physically motivated prescriptions of the mean free path (MFP) of ionizing photons that smoothly attenuate the contribution from distant sources. Implementation of these methods in seminumerical reionization codes requires only modest additional computational effort due to the fact that spatial filtering is still performed on scales larger than the characteristic absorption distance. We find that our smoothly defined MFP prescriptions more effectively suppress largescale structures in the ionization field in seminumerical reionization simulations compared to the standard Rmax approach, and the magnitude of the MFP modulates the power spectrum in a much smoother manner. We show that this suppression of largescale power is significant enough to be relevant for upcoming 21cm power spectrum observations. Finally, we show that in our model, the MFP plays a larger role in regulating the reionization history than in models using Rmax.
more » « less Award ID(s):
 1812458
 NSFPAR ID:
 10367908
 Publisher / Repository:
 Oxford University Press
 Date Published:
 Journal Name:
 Monthly Notices of the Royal Astronomical Society
 Volume:
 514
 Issue:
 1
 ISSN:
 00358711
 Page Range / eLocation ID:
 p. 13021314
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
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