Onset of Cosmic Reionization: Evidence of an Ionized Bubble Merely 680 Myr after the Big Bang

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-α 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.

Texas Spectroscopic Search for Lyα Emission at the End of Reionization I. Constraining the Lyα Equivalent-width Distribution at 6.0 < z < 7.0

“…Spectroscopic confirmation of z > 7 galaxies via Lyα emission implies that these galaxies must be surrounded by ionized bubbles, making them visible in Lyα emission. Re-ferring the models from Yajima et al (2018), and following Tilvi et al (2020), the sizes of individual H II bubbles created by the galaxies could be roughly up to ∼1 pMpc. Specifically, z7 GND 42912 at z = 7.51 (in Pair A) and z7 GND 16863 at z = 7.60 (in Pair B) are massive (M * > 10 9 M ) and bright in their UV (M UV = −21.6 and −21.2), which could be enough to form ∼1 pMpc size ionized bubbles around them.…”

“…With extensive Lyα spectroscopic data of 60 Lyα detected galaxies over a wide field area at z ∼ 6 − 7, Pentericci et al (2018a) suggest a smoother evolution of the IGM compared to previous studies, proposing that the IGM was not fully ionized by z = 6 (see also Kulkarni et al 2019;Fuller et al 2020). Furthermore, while Zheng et al (2017), Castellano et al (2018), and Tilvi et al (2020) report their observations of an ionized bubble via detection of multiple Lyα emitters at z ∼ 7 − 8, non/rare detections of Lyα in Hoag et al (2019) and Mason et al (2019) suggest a significantly neutral fraction in the IGM at z ∼ 7.5, with Hoag et al (2019) reporting a very high neutral fraction of 90% at z ∼ 7.6. Taken together, these results do not tell a coherent story.…”

The size and pervasiveness of Ly α–UV spatial offsets in star-forming galaxies at z ∼ 6

“…The advent of ground-based 8-10-m class telescopes such as the Very Large Telescope array (VLT), Keck, Gemini, and Subaru, extremely sensitive ground-based submillimeter arrays such as the Atacama Large Millimeter/submillimeter Array (ALMA), as well as space-based ⋆ E-mail: blemaux@ucdavis.edu optical/near-infrared facilities such as the Hubble Space Telescope (HST ) and the Spitzer Space Telescope, performing imaging and spectroscopy at a variety of wavelengths have enabled studies of galaxies at increasingly higher redshifts. These studies, especially those over the last decade, have now become powerful enough to begin to explore the epoch known as "reionization" (EoR), during which the first stars, galaxies, and active galactic nuclei (AGN) were beginning to ionize what was a neutral in-tergalactic medium of hydrogen gas (e.g., Treu et al 2013;Tilvi et al 2014Tilvi et al , 2020Schmidt et al 2016;Hashimoto et al 2018aHashimoto et al ,b, 2019Mason et al 2018Mason et al , 2019Hoag et al 2019b;Tamura et al 2019;Fuller et al 2020). It is expected that the very first galaxies started to form when the universe was less than 1 Gyr old (see, e.g., Mawatari et al 2020;Strait et al 2020 and references therein), i.e., at z ∼ > 6, and that these first sources played a crucial role in the reionization that appears to have ended at z∼6 (Becker et al 2001;Fan et al 2006).…”

CHORUS. I. Cosmic HydrOgen Reionization Unveiled with Subaru: Overview

“…Observationally, the spatial inhomogeneity of xHI at z ∼ 6 has been reported to be based on quasar spectra (Becker et al 2015) and LAE LFs (Ouchi et al 2010;Nakamura et al 2011). Obtaining the overdensities of LAEs at z > ∼ 6.5 may support the concept that these LAEs are located in the ionized bubbles produced by the galaxy overdensities (e.g., Castellano et al 2016;Bagley et al 2017;Higuchi et al 2019;Harikane et al 2019;Tilvi et al 2020). Resolving the ionization topology is one of the main scientific goals for future H I 21 cm tomography with the Square Kilometer Array (SKA) (e.g., Hasegawa et al 2016).…”