SILVERRUSH. XI. Constraints on the Ly? Luminosity Function and Cosmic Reionization at z = 7.3 with Subaru/Hyper Suprime-Cam

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