CHORUS II. Subaru/HSC Determination of the Ly$\alpha$ Luminosity Function at $z=7.0$: Constraints on Cosmic Reionization Model Parameter

TL;DR

This study measures the Ly$ extalpha$ luminosity function at redshift 7.0 using Subaru/HSC data, providing precise constraints that inform models of cosmic reionization and showing a monotonic decline in Ly$ extalpha$ luminosity density from redshift 6 to 8.

Contribution

The paper presents the largest and most precise Ly$ extalpha$ LF at z=7.0, clarifying its shape and evolution, and refutes previous claims of a bright-end excess.

Findings

Ly$ extalpha$ LF shape is consistent with a steep slope of -2.5.

No evidence of bright-end excess at z=7.

Ly$ extalpha$ luminosity density declines from z=6 to 8.

Abstract

We present the Ly$\alpha$ luminosity function (LF) derived from 34 Ly$\alpha$ emitters (LAEs) at $z=7.0$ on the sky of $3.1$ deg$^2$, the largest sample compared to those in the literature obtained at a redshift $z\gtrsim7$. The LAE sample is made by deep large-area Subaru narrowband observations conducted by the Cosmic HydrOgen Reionization Unveiled with Subaru (CHORUS) project. The $z=7.0$ Ly$\alpha$ LF of our project is consistent with those of the previous DECam and Subaru studies at the bright and faint ends, respectively, while our $z=7.0$ Ly$\alpha$ LF has uncertainties significantly smaller than those of the previous study results. Exploiting the small errors of our measurements, we investigate the shape of the faint to bright-end Ly$\alpha$ LF. We find that the $z=7.0$ Ly$\alpha$ LF shape can be explained by the steep slope of $\alpha \simeq -2.5$ suggested at $z=6.6$, and that there is no clear signature of a bright-end excess at $z\simeq 7$ claimed by the previous work, which was thought to be made by the ionized bubbles around bright LAEs whose Ly$\alpha$ photons could easily escape from the partly neutral IGM at $z \simeq 7$. We estimate the Ly$\alpha$ luminosity densities (LDs) with Ly$\alpha$ LFs at $z\simeq 6-8$ given by our and the previous studies, and compare the evolution of the UV-continuum LD estimated with dropouts. The Ly$\alpha$ LD monotonically decreases from $z\sim 6$ to $8$, and evolves stronger than the UV-continuum LD, indicative of the Ly$\alpha$ damping wing absorption of the IGM towards the heart of the reionization epoch.