2021ApJ...915...32S

We present spectroscopic confirmation of two new massive galaxy protoclusters at z = 2.24 ± 0.02, BOSS1244 and BOSS1542, traced by groups of Coherently Strong Lyα Absorption (CoSLA) systems imprinted in the absorption spectra of a number of quasars from the Sloan Digital Sky Survey III (SDSS III) and identified as overdensities of narrowband-selected Hα emitters (HAEs). Using Multiple Mirror Telescope/Magellan Infrared Spectrograph and Large Binocular Telescope/LUCI near-infrared (NIR) spectroscopy, we confirm 46 and 36 HAEs in the BOSS1244 (∼55 arcmin²) and BOSS1542 (∼61 arcmin²) fields, respectively. BOSS1244 displays a southwest (SW) component at z = 2.230 ± 0.002 and another northeast (NE) component at z = 2.246 ± 0.001 with the line-of-sight velocity dispersions of 405 ± 202 km s^–1 and 377 ± 99 km s^–1, respectively. Interestingly, we find that the SW region of BOSS1244 contains two substructures in redshift space, likely merging to form a larger system. In contrast, BOSS1542 exhibits an extended filamentary structure with a low-velocity dispersion of 247 ± 32 km s^–1 at z = 2.241 ± 0.001, providing a direct confirmation of a large-scale cosmic web in the early universe. The galaxy overdensities δ_g on the scale of 15 cMpc are 22.9 ± 4.9, 10.9 ± 2.5, and 20.5 ± 3.9 for the BOSS1244 SW, BOSS1244 NE, and BOSS1542 filament, respectively. They are the most overdense galaxy protoclusters (δ_g > 20) discovered to date at z > 2. These systems are expected to become virialized at z ∼ 0 with a total mass of M_SW = (1.59 ± 0.20) x 10¹⁵M_☉, M_NE = (0.83 ± 0.11) x 10¹⁵ M_☉ and M_filament = (1.42 ± 0.18) x 10¹⁵ M_☉, respectively. Our results suggest that the dense substructures of BOSS1244 and BOSS1542 will eventually evolve into the Coma-type galaxy clusters or even larger. Together with BOSS1441 described in Cai et al., these extremely massive overdensities at z = 2-3 exhibit different morphologies, indicating that they are in different assembly stages in the formation of early galaxy clusters. Furthermore, there are two quasar pairs in BOSS1441 and one quasar pair in BOSS1244 and BOSS1542; CoSLAs detected in these quasar pairs can be used to trace the extremely massive large-scale structures of the universe.