Searching for Islands of Reionization: A Potential Ionized Bubble Powered by a Spectroscopic Overdensity at z = 8.7

TL;DR

This study reports the detection of Lyα emission from a galaxy at z=8.665, indicating the presence of large ionized bubbles during early reionization, and suggests an overdense region powering these bubbles.

Contribution

First spectroscopic confirmation of a galaxy within an ionized bubble at z~8.7, linking galaxy overdensity to reionization structures.

Findings

Detected Lyα emission at z=8.665 from a galaxy in the CANDELS EGS field.

Identified a physical proximity of two galaxies with Lyα emission, indicating a large ionized bubble.

Overdensity in the region likely powers the ionized bubble, consistent with previous photometric data.

Abstract

We present the results from a spectroscopic survey using the MOSFIRE near-infrared spectrograph on the 10m Keck telescope to search for Ly$\alpha$ emission from candidate galaxies at z ~9-10 in four of the CANDELS fields (GOODS-N, EGS, UDS, and COSMOS). We observed 11 target galaxies, detecting Ly$\alpha$ from one object in ~8.1 hours of integration, at z = 8.665 +/- 0.001 with an integrated signal-to-noise > 7. This galaxy is in the CANDELS Extended Groth Strip (EGS) field, and lies physically close (3.5 physical Mpc [pMpc]) to another confirmed galaxy in this field with Ly$\alpha$ detected at z=8.683 (Zitrin et al. 2015). The detection of Ly$\alpha$ suggests the existence of large (~1 pMpc) ionized bubbles fairly early in the reionization process. We explore the ionizing output needed to create bubbles of this size at this epoch and find that such a bubble requires more than the ionizing power provided by the full expected population of galaxies (by integrating the UV Luminosity Function down to M_UV = -13). The Ly$\alpha$ we detect would be able to escape the predominantly-neutral intergalactic medium at this epoch if our galaxy is inhabiting an overdensity, which would be consistent with the photometric overdensity previously identified in this region by Finkelstein et al. 2021. This implies that the CANDELS EGS field is hosting an overdensity at z = 8.7 which is powering one (or more) ionized bubbles, a hypothesis that will be imminently testable with forthcoming James Webb Space Telescope observations in this field.