We present a narrowband survey with three adjacent filters for z=2.8--2.9
Lyman Alpha Emitter (LAE) galaxies in the Extended Chandra Deep Field South
(ECDFS), along with spectroscopic followup. With a complete sample of 96 LAEs
in the narrowband NB466, we confirm a large-scale structure at z~ 2.8. Compared
to the blank field in NB470 and NB475, the LAE density excess in the NB466
field is ~6.0+/-0.8 times the standard deviation expected at z~2.8, assuming a
linear bias of 2. The overdense large scale structure in NB466 can be
decomposed into 4 protoclusters, whose overdensities are 4.6 - 6.6. These 4
protoclusters are expected to evolve into a Coma-like cluster at z~ 0. In the
meanwhile, we investigate the average star-formation rates derived from
Ly\alpha, rest-frame UV and X-ray, the Ly\alpha luminosity functions, the
Ly\alpha photon densities and their dependence on the environment. We find
that the Ly\alpha photon density in the overdense field (NB466) is ~50\%
higher than that in the blank field (NB470+NB475). The 3 brightest LAEs,
including a quasar at z=2.81, are all detected in X-ray and in NB466. These
three LAE-AGNs contribute an extra 20--30\% Ly\alpha photon density.
Furthermore, we find that LAEs in overdense regions are younger and less dusty.
We conclude that the structure we found is a significant and rare density peak,
and narrowband imaging is an efficient method to detect and study such
structures in the high-z universe.