Lyα EMITTER GALAXIES AT z ∼ 2.8 IN THE EXTENDED CHANDRA DEEP FIELD SOUTH. I. TRACING THE LARGE-SCALE STRUCTURE VIA Lyα IMAGING ∗

Abstract

We present a narrowband survey with three adjacent filters for z  = 2.8–2.9 Lyman alpha (Ly α) 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 arcmin 2 ) 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 four protoclusters, whose overdensities (each within an equivalent comoving volume 15 3 Mpc 3 ) 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  ≥ 10 15 M ⊙ ) 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 Ly α , rest-frame UV, and X-ray bands are ∼4, 10, and <16 M ⊙ yr −1 , respectively, implying a Ly α escape fraction of 25% ≲   ≲ 40% and a UV continuum escape fraction of  ≳ 62% for LAEs at z ∼ 2.8. The Ly α photon density calculated from the integrated Ly α 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% Ly α 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 (∼2–3 σ ) 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.