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