Ly\u2009α as a tracer of cosmic reionization in the SPHINX radiation-hydrodynamics cosmological simulation

Abstract

The Lyα emission line is one of the most promising probes of cosmic reionisation but isolating the signature of a change in the ionisation state of the IGM is challenging because of intrinsic evolution and internal radiation transfer effects. We present the first study of the evolution of Lyα emitters (LAE) during the epoch of reionisation based on a full radiation-hydrodynamics cosmological simulation that is able to capture both the large-scale process of reionisation and the small-scale properties of galaxies. We predict the Lyα emission of galaxies in the 103 cMpc3 sphinx simulation at 6 ≤ z ≤ 9 by computing the full Lyα radiation transfer from ISM to IGM scales. sphinx is able to reproduce many observational constraints such as the UV/Lyα luminosity functions and stellar mass functions at z & 6 for the dynamical range probed by our simulation (M1500 & −18, LLyα . 1042 erg s−1, M★ . 109M ). As intrinsic Lyα emission and internal Lyα escape fractions barely evolve from z = 6 to 9, the observed suppression of Lyα luminosities with increasing redshift is fully attributed to IGM absorption. For most observable galaxies (M1500 . −16), the Lyα line profiles are slightly shifted to the red due to internal radiative transfer effects which mitigates the effect of IGM absorption. Overall, the enhanced Lyα suppression during reionisation traces the IGM neutral fraction xHI well but the predicted amplitude of this reduction is a strong function of the Lyα peak shift, which is set at ISM/CGM scales. We find that a large number of LAEs could be detectable in very deep surveys during reionisation when xHI is still ≈ 50%.