Lyα EMITTER GALAXIES at z ∼ 2.8 in the EXTENDED CHANDRA DEEP FIELD SOUTH. I. TRACING the LARGE-SCALE STRUCTURE VIA Lyα IMAGING

We present a narrowband survey with three adjacent filters for z =2.8-2.9 Lyman alpha (Lyα) emitter (LAE) galaxies in the Extended Chandra Deep Field South (ECDFS), along with spectroscopic follow-up. With a complete sample of 96 LAE candidates in the narrowband NB466, we confirm the large-scale structure at z ∼ 2.8 suggested by previous spectroscopic surveys. Compared to the blank field detected with the other two narrowband filters NB470 and NB475, the LAE-density excess in NB466 (900 arcmin²) is ∼ 6.00.8 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 four protoclusters, whose overdensities (each within an equivalent comoving volume 15³ Mpc³) relative to the blank field (NB470+NB475) are in the range of 4.6-6.6. These four protoclusters are expected to evolve into a Coma-like cluster (M ≥ 10¹⁵M_o) at z ∼ 0. We also investigate the various properties of LAEs at z = 2.8-2.9 and their dependence on the environment. The average star formation rates derived from the Lyα, rest-frame UV, and X-ray bands are ∼4, 10, and <16 M _o yr^-1, respectively, implying a Lyα escape fraction of 25% ≲ ≲ 40% and a UV continuum escape fraction of 62% for LAEs at z ∼ 2.8. The Lyα photon density calculated from the integrated Lyα luminosity function in the overdense field (NB466) is ∼50% higher than that in the blank field (NB470+NB475), and more bright LAEs are found in the overdense field. The three brightest LAEs, including a quasar at z = 2.81, are all detected in the X-ray band and in NB466. These three LAE-active galactic nuclei contribute an extra 20%-30% Lyα photon density compared to other LAE galaxies. Furthermore, we find that LAEs in overdense regions have larger equivalent width values, bluer U - B and V - R (∼2-3σ) colors compared with those in lower density regions, indicating that LAEs in overdense regions are younger and possible less dusty. We conclude that the structure at z ∼ 2.8 in the ECDFS field is a very significant and rare density peak similar to the SSA22 protocluster, and narrowband imaging is an efficient method of detecting and studying such structures in the high-z universe.