The Reionization and Galaxy Evolution Probed by z=7 Lyman Alpha Emitters

TL;DR

This study used narrowband imaging and spectroscopy to identify z=7 Lyman alpha emitters, revealing a significant decline in their density at high redshift, which suggests ongoing cosmic reionization and galaxy evolution.

Contribution

First detection of a z=6.96 Lyman alpha emitter in the Subaru Deep Field, providing new insights into galaxy formation shortly after the Big Bang.

Findings

LAE density drops to 17% at z=7 compared to z=6.6

Galaxy evolution alone cannot explain the decline in LAE density

Rapid increase in neutral hydrogen fraction likely causes Lyman alpha attenuation

Abstract

We made a narrowband NB973 (bandwidth of 200A at 9755A) imaging of the Subaru Deep Field (SDF) and found two z=7 Lyman alpha emitter (LAE) candidates down to NB973=24.9. Carrying out deep follow-up spectroscopy, we identified one of them as a real z=6.96 LAE. This has shown that galaxy formation was in progress just 750 Myr after the Big Bang. Meanwhile, the Lyman alpha line luminosity function of LAE is known to decline from z=5.7 to 6.6 in the SDF. L* at z=6.6 is 40-60% of that at z=5.7. We also confirm that the number density of z=7 LAE is only 17% of the density at z=6.6 comparing the latest SDF LAE samples. This series of significant decreases in LAE density with increasing redshift can be the result of galaxy evolution during these epochs. However, using the UV continuum luminosity functions of LAEs, those of Lyman break galaxies, and a LAE evolution model based on the hierarchical clustering, we find that galaxy evolution alone cannot explain all the decrease in density. This extra density deficit can be interpreted as the attenuation of the Lyman alpha photons from LAEs due to a rapid evolution of neutral hydrogen fraction during the ongoing cosmic reionization at z~6.6-7.