Spectroscopic investigation of a reionized galaxy overdensity at z=7
  We present deep spectroscopic follow-up observations of the Bremer Deep Field
(BDF) where the two $z\sim$7 bright Ly$\alpha$ emitters (LAE) BDF521 and
BDF3299 were previously discovered by Vanzella et al. (2011) and where a factor
of $\sim$3-4 overdensity of faint LBGs has been found by Castellano et al.
(2016). We confirm a new bright Ly$\alpha$ emitter, BDF2195, at the same
redshift of BDF521, $z=7.008$, and at only $\sim$90 kpc physical distance from
it, confirming that the BDF area is likely an overdense, reionized region. A
quantitative assessment of the Ly$\alpha$ fraction shows that the number of
detected bright emitters is much higher than the average found at z$\sim$7,
suggesting a high Ly$\alpha$ transmission through the inter-galactic medium
(IGM). However, the line visibility from fainter galaxies is at odds with this
finding, since no Ly$\alpha$ emission is found in any of the observed
candidates with $M_{UV}>$-20.25. This discrepancy can be understood either if
some mechanism prevents Ly$\alpha$ emission from fainter galaxies within the
ionized bubbles from reaching the observer, or if faint galaxies are located
outside the reionized area and bright LAEs are solely responsible for the
creation of their own HII regions. A thorough assessment of the nature of the
BDF region and of its sources of re-ionizing radiation will be made possible by
JWST spectroscopic capabilities.
