The BlueTides simulation: first galaxies and reionization

The authors 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 × 70403 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 ∼ 8–10. The authors find good agreement between observations and the predicted galaxy luminosity function in the currently observable range −18 ≤ MUV ≤ −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 α ∼ −1.8 at z ∼ 8 and redshift dependence of α ∼ (1 + z)−0.4. The AGN population has a luminosity function well fit by a power law with a slope α ∼ −2.4 that compares favourably with the deepest CANDELS GOODS fields. The authors investigate how these luminosity functions affect the progress of reionization, and find that a high Lyman α escape fraction (fesc ∼ 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 MUV = −12) which results in a rapid reionization, disfavoured by current observations.