# The Ly$\alpha$ Luminosity Function and Cosmic Reionization at $z \sim$ 7.0: a Tale of Two LAGER Fields

## TL;DR

This study analyzes the Ly$ ext{alpha}$ luminosity function at redshift 7.0 using a large sample of emitters, revealing evidence for patchy reionization and large-scale overdensities, and estimating the neutral hydrogen fraction during the epoch of reionization.

## Contribution

It provides the largest sample of Ly$ ext{alpha}$ emitters at $z\, ext{sim}\,7.0$, identifies large-scale overdensities, and offers new insights into the reionization topology and neutral hydrogen fraction.

## Key findings

- Detection of two large-scale overdense regions likely protoclusters.
- Confirmation of a bright-end bump in the luminosity function in COSMOS.
- Estimated neutral hydrogen fraction of 0.2–0.4 at $z\, ext{sim}\,7.0$.

## Abstract

We present the largest-ever sample of 79 Ly$\alpha$ emitters (LAEs) at $z\sim$ 7.0 selected in the COSMOS and CDFS fields of the LAGER project (the Lyman Alpha Galaxies in the Epoch of Reionization). Our newly amassed ultradeep narrowband exposure and deeper/wider broadband images have more than doubled the number of LAEs in COSMOS, and we have selected 30 LAEs in the second field CDFS. We detect two large-scale LAE-overdense regions in the COSMOS that are likely protoclusters at the highest redshift to date. We perform injection and recovery simulations to derive the sample incompleteness. We show significant incompleteness comes from blending with foreground sources, which however has not been corrected in LAE luminosity functions in {the} literature. The bright end bump in the Ly$\alpha$ luminosity function in COSMOS is confirmed with 6 (2 newly selected) luminous LAEs (L$_{Ly\alpha}$ $>$ 10$^{43.3}$ erg s$^{-1}$). Interestingly, the bump is absent in CDFS, in which only one luminous LAE is detected. Meanwhile, the faint end luminosity functions from the two fields well agree with each other. The 6 luminous LAEs in COSMOS coincide with 2 LAE-overdense regions, while such regions are not seen in CDFS. The bright-end luminosity function bump could be attributed to ionized bubbles in a patchy reionization. It appears associated with cosmic overdensities, thus supports an inside-out reionization topology at $z$ $\sim$ 7.0, i.e., the high density peaks were ionized earlier compared to the voids. An average neutral hydrogen fraction of $x_{HI}$ $\sim$ 0.2 -- 0.4 is derived at $z\sim$ 7.0 based on the cosmic evolution of the Ly$\alpha$ luminosity function.