AMBER: A Semi-numerical Abundance Matching Box for the Epoch of Reionization

The Abundance Matching Box for the Epoch of Reionization (AMBER) is a semi-numerical code for modeling the cosmic dawn. The new algorithm is not based on the excursion set formalism for reionization, but takes the novel approach of calculating the reionization-redshift field ${z}_{\mathrm{re}}({\boldsymbol{x}})$ assuming that hydrogen gas encountering higher radiation intensity are photoionized earlier. Redshift values are assigned while matching the abundance of ionized mass according to a given mass-weighted ionization fraction ${\bar{x}}_{{\rm{i}}}(z)$ . The code has the unique advantage of allowing users to directly specify the reionization history through the redshift midpoint z _mid, duration Δ_z, and asymmetry A _z input parameters. The reionization process is further controlled through the minimum halo mass M _min for galaxy formation and the radiation mean free path l _mfp for radiative transfer. We implement improved methods for constructing density, velocity, halo, and radiation fields, which are essential components for modeling reionization observables. We compare AMBER with two other semi-numerical methods and find that our code more accurately reproduces the results from radiation-hydrodynamic simulations. The parallelized code is over four orders of magnitude faster than radiative transfer simulations and will efficiently enable large-volume models, full-sky mock observations, and parameter-space studies. AMBER will be made publicly available to facilitate and transform studies of the Epoch of Reionization.