Onset of Cosmic Reionization: Evidence of An Ionized Bubble Merely 680 Myrs after the Big Bang

TL;DR

This study provides the first spectroscopic confirmation of galaxies at redshift 7.7, revealing an ionized bubble only 680 million years after the Big Bang, which offers new insights into the inhomogeneous cosmic reionization process.

Contribution

It reports the first spectroscopic detection of galaxies at z=7.7 and demonstrates the formation of a large ionized bubble, highlighting the role of faint galaxies in cosmic reionization.

Findings

Confirmed two galaxies at z=7.7 within 680 Myrs after Big Bang.

Estimated ionized bubble radius of about 1.02 Mpc.

Faint galaxies significantly contribute to reionization.

Abstract

While most of the inter-galactic medium (IGM) today is permeated by ionized hydrogen, it was largely filled with neutral hydrogen for the first 700 million years after the Big Bang. The process that ionized the IGM (cosmic reionization) is expected to be spatially inhomogeneous, with fainter galaxies playing a significant role. However, we still have only a few direct constraints on the reionization process. Here we report the first spectroscopic confirmation of two galaxies and very likely a third galaxy in a group (hereafter EGS77) at redshift z = 7.7, merely 680 Myrs after the Big Bang. The physical separation among the three members is < 0.7 Mpc. We estimate the radius of ionized bubble of the brightest galaxy to be about 1.02 Mpc, and show that the individual ionized bubbles formed by all three galaxies likely overlap significantly, forming a large yet localized ionized region, which leads to the spatial inhomogeneity in the reionization process. It is striking that two of three galaxies in EGS77 are quite faint in the continuum, thanks to our selection of reionizing sources using their Lyman-alpha line emission. Indeed, one is the faintest spectroscopically confirmed galaxy yet discovered at such high redshifts. Our observations provide direct constraints in the process of cosmic reionization, and allow us to investigate the properties of sources responsible for reionizing the universe.