LAGER Ly\(\alpha\) Luminosity Function at \(z\sim7\), Implications for Reionization

Abstract

We present a new measurement of the Ly\(\alpha\) luminosity function at redshift \(z=6.9\), finding moderate evolution from \(z=5.7\) that is consistent with a fully or largely ionized \(z\sim7\) intergalactic medium. Our result is based on four fields of the LAGER (Lyman Alpha Galaxies in the Epoch of Reionization) project. Our survey volume of \(6.1\times10^{6}\) Mpc\(^{3}\) is double that of the next largest \(z\sim 7\) survey. We combine two new LAGER fields (WIDE12 and GAMA15A) with two previously reported LAGER fields (COSMOS and CDFS). In the new fields, we identify \(N=95\) new \(z=6.9\) Ly\(\alpha\) emitters (LAEs); characterize our survey's completeness and reliability; and compute Ly\(\alpha\) luminosity functions. The best-fit Schechter luminosity function parameters for all four LAGER fields are in good general agreement. Two fields (COSMOS and WIDE12) show evidence for a bright-end excess above the Schechter function fit. We find that the Ly\(\alpha\) luminosity density declines at the same rate as the UV continuum LF from \(z=5.7\) to \(z=6.9\). This is consistent with an intergalactic medium that was fully ionized as early as redshift \(z\sim 7\), or with a volume-averaged neutral hydrogen fraction of \(x_{HI} < 0.33\) at \(1\sigma\).