SILVERRUSH XI: Constraints on the Ly\(\alpha\) luminosity function and cosmic reionization at \(z=7.3\) with Subaru/Hyper Suprime-Cam

Abstract

The Ly\(\alpha\) luminosity function (LF) of Ly\(\alpha\) emitters (LAEs) has been used to constrain the neutral hydrogen fraction in the intergalactic medium (IGM) and thus the timeline of cosmic reionization. Here we present the results of a new narrow-band imaging survey for \(z=7.3\) LAEs in a large area of \(\sim 3\ \mathrm{deg}^2\) with Subaru/Hyper Suprime-Cam. No LAEs are detected down to \(L_{\mathrm{Ly}\alpha}\simeq 10^{43.2}\ \mathrm{erg\ s^{-1}}\) in an effective cosmic volume of \(\sim 2\times 10^6\) Mpc\(^3\), placing an upper limit to the bright part of the \(z=7.3\) Ly\(\alpha\) LF for the first time and confirming a decrease in bright LAEs from \(z=7.0\). By comparing this upper limit with the Ly\(\alpha\) LF in the case of the fully ionized IGM, which is predicted using an observed \(z=5.7\) Ly\(\alpha\) LF on the assumption that the intrinsic Ly\(\alpha\) LF evolves in the same way as the UV LF, we obtain the relative IGM transmission \(T^\mathrm{IGM}_{\mathrm{Ly}\alpha}(7.3)/T^\mathrm{IGM}_{\mathrm{Ly}\alpha}(5.7)<0.77\), and then the volume-averaged neutral fraction \(x_\mathrm{HI}(7.3)>0.28\). Cosmic reionization is thus still ongoing at \(z=7.3\), being consistent with results from other \(x_\mathrm{HI}\) estimation methods. A similar analysis using literature Ly\(\alpha\) LFs finds that at \(z=6.6\) and 7.0 the observed Ly\(\alpha\) LF agrees with the predicted one, consistent with full ionization.