Constraints on the Epoch of Reionization with Roman Space Telescope and the Void Probability Function of Lyman-Alpha Emitters

Abstract

We use large simulations of Lyman-Alpha Emitters with different fractions of ionized intergalactic medium to quantify the clustering of Ly$\alpha$ 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. Since Roman will be able to carry out blind spectroscopic surveys of Ly$\alpha$ emitters continuously between $7<z<12$ to sensitivities of at least $10^{-17}$ erg sec$^{-1}$ 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 deg$^2$, and quantify what constraints on reionization the VPF may find for these surveys. A survey of 1 deg$^2$ would distinguish between very late reionization and early reionization to 3$\sigma$ near $z=7.7$ with the VPF. The VPF of a 4 deg$^2$ survey can distinguish between slow vs.\ fast, and early vs.\ late, reionization at $> 3-4\sigma$ at several redshifts between $7<z<9$. However, a survey of 13-16 deg$^2$ would allow the VPF to give several robust constraints ($>5-8\sigma$) across the epoch of reionization, and would yield a detailed history of the reionization of the IGM and its effect on Lyman-$\alpha$ Emitter clustering.