Constraints on the Epoch of Reionization with Roman Space Telescope and the Void Probability Function of Lyα Emitters

Abstract

We use large simulations of Lyα emitters with different fractions of ionized intergalactic medium to quantify the clustering of Lyα emitters as measured by the void probability function (VPF), and how it evolves under different ionization scenarios. We quantify how well we might be able to distinguish between these scenarios with a deep spectroscopic survey using the future Nancy Grace Roman Space Telescope. Because Roman will be able to carry out blind spectroscopic surveys of Lyα emitters continuously between 7 < z < 12 to sensitivities of at least 10−17 erg s−1 over a wide field of view, it can measure the epoch of reionization as well as the pace of ionization of the intergalactic medium (IGM). We compare deep Roman surveys covering roughly 1, 4, and 16 deg2, and quantify what constraints on reionization the VPF may find for these surveys. A survey of 1 deg2 would distinguish between very late reionization and early reionization to 3σ near z = 7.7 with the VPF. The VPF of a 4 deg2 survey can distinguish between slow versus fast, as well as early versus late, reionization at >3σ–4σ at several redshifts between 7 < z < 9. However, a survey of 13–16 deg2 would allow the VPF to give several robust constraints (>5–8σ) across the epoch of reionization, and would yield a detailed history of the reionization of the IGM and its effect on Lyα emitter clustering.