The ASTRID Simulation: Galaxy Formation and Reionization

We introduce the ASTRID simulation, a large-scale cosmological hydrodynamic simulation in a $250$ Mpc/h box with $2\times 5500^3$ particles. ASTRID contains a large number of high redshift galaxies, which can be compared to future survey data, and resolves galaxies in halos more massive than $2\times 10^9 M_\odot$. ASTRID has been run from $z=99$ to $z=3$. As a particular focus is modelling the high redshift Universe, it contains models for inhomogeneous hydrogen and helium reionization, baryon relative velocities and massive neutrinos, as well as supernova and AGN feedback. The black hole model includes mergers driven by dynamical friction rather than repositioning. We briefly summarise the implemented models, and the technical choices we took when developing the simulation code. We validate the model, showing good agreement with observed UV luminosity functions, galaxy stellar mass functions and specific star formation rates. We show that the redshift at which a given galaxy underwent hydrogen reionization has a large effect on the halo gas fraction. Finally, at $z=6$, halos with $M \sim 2\times 10^9 M_\odot$ which have been reionized have a star formation rate $1.5$ times greater than those which have not yet been reionized.