The Prevalence of Galaxy Overdensities Around UV-Luminous Lyman $\mathbf{\alpha}$ Emitters in the Epoch of Reionization

TL;DR

This study finds that UV-luminous Ly$ extalpha$ emitters during the epoch of reionization are surrounded by significant galaxy overdensities, supporting models where ionized bubbles form in dense regions, with implications for understanding reionization.

Contribution

The paper provides observational evidence of galaxy overdensities around high-redshift Ly$ extalpha$ emitters, confirming theoretical predictions about ionized bubble formation during reionization.

Findings

Overdensities of 3-9 times the expected galaxy number around luminous Ly$ extalpha$ sources.

Consistent results with the Dragons simulation predictions.

Ubiquitous overdensities around Ly$ extalpha$ emitters at $z extgreater6$.

Abstract

Before the end of the epoch of reionization, the Hydrogen in the Universe was predominantly neutral. This leads to a strong attenuation of Ly$\alpha$ lines of $z\gtrsim6$ galaxies in the intergalactic medium. Nevertheless, Ly$\alpha$ has been detected up to very high redshifts ($z\sim9$) for several especially UV luminous galaxies. Here, we test to what extent the galaxy's local environment might impact the Ly$\alpha$ transmission of such sources. We present an analysis of dedicated Hubble Space Telescope (HST) imaging in the CANDELS/EGS field to search for fainter neighbours around three of the most UV luminous and most distant spectroscopically confirmed Ly$\alpha$ emitters: EGS-zs8-1, EGS-zs8-2 and EGSY-z8p7 at $z_\mathrm{spec}=7.73$, 7.48, and 8.68, respectively. We combine the multi-wavelength HST imaging with Spitzer data to reliably select $z\sim7-9$ galaxies around the central, UV-luminous sources. In all cases, we find a clear enhancement of neighbouring galaxies compared to the expected number in a blank field (by a factor $\sim 3-9\times$). Our analysis thus reveals ubiquitous overdensities around luminous Ly$\alpha$ emitting sources in the heart of the cosmic reionization epoch. We show that our results are in excellent agreement with expectations from the Dragons simulation, confirming the theoretical prediction that the first ionized bubbles preferentially formed in overdense regions. JWST follow-up observations of the neighbouring galaxies identified here will be needed to confirm their physical association and to map out the ionized regions produced by these sources.