Efficient simulation of discrete galaxy populations and associated radiation fields over the first billion years

Understanding the epochs of cosmic dawn and reionisation requires us to leverage multi-wavelength and multi-tracer observations, with each dataset providing a complementary piece of the puzzle. To interpret these data, we updated the public simulation code, 21cmFASTv4, to include a discrete source model based on stochastic sampling of conditional mass functions and semi-empirical galaxy relations. We demonstrate that our new galaxy model, which parametrises the means and scatters of well-established scaling relations, is flexible enough to characterise a range of predictions from different hydrodynamic cosmological simulations of high-redshift galaxies. Combining a discrete galaxy population with approximate, efficient radiative transfer allows us to self-consistently forward-model galaxy surveys, line intensity maps (LIMs), and observations of the intergalactic medium (IGM). Not only does each observable probe different scales and physical processes, but their cross-correlation will maximise the information gained from each measurement by probing the galaxy-IGM connection at high redshift. In this work, we found that a stochastic source field produces significant shot-noise in 21cm and LIM power spectra. Scatter in galaxy properties can be constrained using ultraviolet (UV) luminosity functions and/or 21cm power spectra, especially if astrophysical scatter is higher than expected (as might be needed to explain recent JWST observations). Our modelling pipeline is both flexible and computationally efficient, thereby facilitating high-dimensional, multi-tracer, field-level Bayesian inference of cosmology and astrophysics over the first billion years.