The BlueTides simulation: first galaxies and reionization

Abstract

We introduce the BlueTides simulation and report initial results for the luminosity functions of the first galaxies and AGN, and their contribution to reionization. BlueTides was run on the BlueWaters cluster at NCSA from $z=99$ to $z=8.0$ and includes 2$\times$7040$^3$ particles in a $400$Mpc/h 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. The star formation rate density in the simulation is a good match to current observational data at $z\sim 8-10$. We find good agreement between observations and the predicted galaxy luminosity function in the currently observable range $-18\le M_{\mathrm UV} \le -22.5$ with some dust extinction required to match the abundance of brighter objects. BlueTides implements a patchy reionization model that produces a fluctuating UV background. BlueTides predicts number counts for galaxies fainter than current observational limits which are consistent with extrapolating the faint end slope of the luminosity function with a power law index $\alpha\sim -1.8$ at $z\sim 8$ and redshift dependence of $\alpha\sim (1+z)^{-0.4}$. The AGN population has a luminosity function well fit by a power law with a slope $\alpha\sim -2.4$ 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 ($f_\mathrm{esc} \sim 0.5$) is required if galaxies dominate the ionising photon budget during reionization. Smaller galaxy escape fractions imply a large contribution from faint AGN (down to $M_\mathrm{UV}=-12$) which results in a rapid reionization, disfavoured by current observations.