Introducing the Lumina project: large-volume radiation-hydrodynamic simulations of the epochs of hydrogen and helium reionization

Abstract: Understanding how galaxies and active galactic nuclei (AGN) jointly drive the reionization of the intergalactic medium (IGM) across cosmic time remains a major challenge in cosmology. We present Lumina, a large-volume radiation-hydrodynamic simulation that self-consistently follows the coupled evolution of the intergalactic medium, galaxies, and AGN through HI, HeI, and HeII reionization down to redshift $z=3$. Lumina evolves a cosmological volume of comoving side length $L_{\mathrm{box}}=500\,\mathrm{cMpc}$ with $2\times 6000^{3}$ resolution elements, corresponding to baryonic and dark-matter mass resolutions of $3.6\times 10^{6}\,\text{M}_{\odot}$ and $1.9\times 10^{7}\,\text{M}_{\odot}$, respectively. The simulation uses the moving-mesh code AREPO, combining the IllustrisTNG galaxy-formation model with a GPU-accelerated M1 radiation-transport solver in six frequency bins. The initial conditions employ separate transfer functions for baryons and dark matter and include their relative streaming velocity. Lumina predicts a late, predominantly stellar-driven hydrogen reionization, with the median sub-volume fully ionized by $z\approx 5.2$ and residual neutral HI patches persisting until $z\approx 4.75$. HeII reionization is driven self-consistently by AGN and is nearly complete by $z=3$. The simulation yields a Thomson-scattering optical depth in excellent agreement with Planck, an IGM thermal history and photoionization background broadly consistent with observational constraints, and a clear late-time thermal boost associated with HeII reionization. Its galaxy population remains consistent with the original IllustrisTNG project, while the larger volume improves statistics for rare objects, large-scale environments, and cosmic variance, enabling forward modelling of observables linking HI and HeII topologies to the evolving galaxy and AGN populations.