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 similar to 7 bright Ly alpha emitters (LAE) BDF521 and BDF3299 were previously discovered by Vanzella et al. and where a factor of similar to 3-4 overdensity of faint LBGs has been found by Castellano et al. We confirm a new bright Ly alpha emitter, BDF2195, at the same redshift of BDF521, z = 7.008 and at only similar to 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 similar to 7, suggesting a high Ly alpha transmission through the intergalactic medium. However, the line visibility from fainter galaxies is at odds with this finding, as 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 H II regions. A thorough assessment of the nature of the BDF region and of its sources of re-ionizing radiation will be made possible by James Webb Space Telescope spectroscopic capabilities.