The BlueTides simulation: first galaxies and reionization

We introduce the BlueTides simulation and report initial results for the luminosity functions of the first galaxies and active galactic nuclei (AGN), and their contribution to reionization. BlueTides was run on the BlueWaters cluster at National Center for Super-computing Applications from z = 99 to 8.0 and includes 2 x 7040(3) particles in a 400 h(-1) Mpc per side box, making it the largest hydrodynamic simulation ever performed at high redshift. BlueTides includes a pressure-entropy formulation of smoothed particle hydrodynamics, gas cooling, star formation (including molecular hydrogen), black hole growth and models for stellar and AGN feedback processes, and a fluctuating ultraviolet background from a patchy reionization model. The predicted star formation rate density is a good match to current observational data at z similar to 8-10. We find good agreement between observations and the predicted galaxy luminosity function in the currently observable range -18 <= M-UV <=-22.5 with some dust extinction required to match the abundance of brighter objects. The predicted number counts for galaxies fainter than current observational limits are consistent with extrapolating the faint-end slope of the luminosity function with a power-law index alpha similar to -1.8 at z similar to 8 and redshift dependence of alpha similar to (1 + z)(-0.4). The AGN population has a luminosity function well fit by a power law with a slope alpha similar to -2.4 that compares favourably with the deepest CANDELS GOODS fields. We investigate how these luminosity functions affect the progress of reionization, and find that a high Lyman alpha escape fraction (f(esc) similar to 0.5) is required if galaxies dominate the ionizing photon budget during reionization. Smaller galaxy escape fractions imply a large contribution from faint AGN (down to M-UV = -12) which results in a rapid reionization, disfavoured by current observations.