SILVERRUSH. IV. Ly$\hat{I}\pm$ Luminosity Functions at $z=5.7$ and $6.6$ Studied with $\sim$ 1,300 LAEs on the $14-21$ deg${}^2$ Sky

Abstract

We present the Ly$\hat{I}\pm$ luminosity functions (LFs) at $z=$5.7 and 6.6 derived from a new large sample of 1,266 Ly$\hat{I}\pm$ emitters (LAEs) identified in total areas of 14 and 21 deg${}^2$, respectively, based on the early narrowband data of the Subaru/Hyper Suprime-Cam (HSC) survey. Together with careful Monte-Carlo simulations that account for the incompleteness of the LAE selection and the flux estimate systematics in the narrowband imaging, we have determined the Ly$\hat{I}\pm$ LFs with the unprecedentedly small statistical and systematic uncertainties in a wide Ly$\hat{I}\pm$ luminosity range of $10^{42.8-43.8}$ erg s${}^{-1}$. We obtain the best-fit Schechter parameters of $L_{\mathrm{Lya}}^{\ast }=1.6_{-0.6}^{+2.2}(1.7_{-0.7}^{+0.3})\times 10^{43}$ erg s${}^{-1}$, $\ddot{I}{•}_{\mathrm{Lya}}^{\ast }=0.85_{-0.77}^{+1.87}(0.47_{-0.44}^{+1.44})\times 10^{-4}$ Mpc${}^{-3}$, and $\hat{I}\pm =-2.6_{-0.4}^{+0.6}(-2.5_{-0.5}^{+0.5})$ at $z=5.7$ ($6.6$). We confirm that our best-estimate Ly$\hat{I}\pm$ LFs are consistent with the majority of the previous studies, but find that our Ly$\hat{I}\pm$ LFs do not agree with the high number densities of LAEs recently claimed by Matthee/Santos et al.'s studies that may overcorrect the incompleteness and the flux systematics. Our Ly$\hat{I}\pm$ LFs at $z=5.7$ and $6.6$ show an indication that the faint-end slope is very steep ($\hat{I}\pm \simeq -2.5$), although it is also possible that the bright-end LF results are enhanced by systematic effects such as the contribution from AGNs, blended merging galaxies, and/or large ionized bubbles around bright LAEs. Comparing our Ly$\hat{I}\pm$ LF measurements with four independent reionization models, we estimate the neutral hydrogen fraction of the IGM to be $x_{\mathrm{HI}}=0.3\pm 0.2$ at $z=$6.6 that is consistent with the small Thomson scattering optical depth obtained by Planck 2016.

21 pages, 14 figures, accepted for publication in PASJ special issue