2017ApJ...845..172B

Bright Lyα blobs (LABs)-extended nebulae with sizes of ∼100 kpc and Lyα luminosities of ∼10⁴⁴ erg s^–1-often reside in overdensities of compact Lyα emitters (LAEs) that may be galaxy protoclusters. The number density, variance, and internal kinematics of LABs suggest that they themselves trace group-like halos. Here, we test this hierarchical picture, presenting deep, wide-field Lyα narrowband imaging of a 1° x 0.°5 region around a LAB pair at z = 2.3 discovered previously by a blind survey. We find 183 Lyα emitters, including the original LAB pair and three new LABs with Lyα luminosities of (0.9–1.3)×10⁴³ erg s^–1 and isophotal areas of 16-24 arcsec². Using the LAEs as tracers and a new kernel density estimation method, we discover a large-scale overdensity (Bootes J1430+3522) with a surface density contrast of δ_Σ=2.7, a volume density contrast of δ ∼10.4, and a projected diameter of ≃20 comoving Mpc. Comparing with cosmological simulations, we conclude that this LAE overdensity will evolve into a present-day Coma-like cluster with log(M/M_☉) ∼ 15.1 ± 0.2. In this and three other wide-field LAE surveys re-analyzed here, the extents and peak amplitudes of the largest LAE overdensities are similar, not increasing with survey size, and implying that they were indeed the largest structures then and today evolve into rich clusters. Intriguingly, LABs favor the outskirts of the densest LAE concentrations, i.e., intermediate LAE overdensities of δ_Σ=1–2. We speculate that these LABs mark infalling protogroups being accreted by the more massive protocluster.