A Lya protocluster at redshift 7

Lyman alpha emitting galaxies (LAEs) are excellent tracers of large scale structures and can be efficiently selected with narrowband technique at very high redshift. Many protoclusters have been discovered with LAEs at redshift up to 6.6. In this talk, I will present a protocluster at redshift 7 discovered by the LAGER (Lyman Alpha Galaxies in the Epoch of Reionization) project. We used the Dark Energy Camera on CTIO Blanco 4m telescope with a custom-made narrowband filter NB964 to search for LAEs at redshift 7. In the LAGER-COSMOS field, we discovered a protocluster with a significant overdensity of 5.1 and mass of 3.7e15 solar masses. It has been spectroscopically confirmed with Magellan Telescope observations. Furthermore, we found its member galaxies are able to provide sufficient ionizing photons to ionized the IGM within the protocluster. Note the average neutral hydrogen fraction of IGM is still 0.2–0.4 at redshift 7. Thus, this protocluster provides a unique natural laboratory to investigate the environment effect on the reionization and the structure assembling in the early universe.