2021MNRAS.500.4354Z

Massive galaxy overdensities at the peak epoch of cosmic star formation provide ideal testbeds for the formation theories of galaxies and large-scale structure. We report the confirmation of two massive galaxy overdensities at z = 2.24, BOSS1244 and BOSS1542, selected from the Mapping the Most Massive Overdensities Through Hydrogen (MAMMOTH) project using Lyα absorption from the intergalactic medium over the scales of 15-30 h^–1 Mpc imprinted on the quasar spectra. We use Hα emitters (HAEs) as the density tracer and identify them using deep narrow-band H₂S(1) and broad-band K_s imaging data obtained with the wide-field infrared camera (WIRCam) at the Canada-France-Hawaii Telescope. In total, 244 and 223 line emitters are detected in these two fields, and 196 ± 2 and 175 ± 2 are expected to be HAEs with an Hα flux of >2.5 x 10^–17 erg s^–1 cm^–2 (corresponding to a star formation rate of >5 M_☉ yr^–1). The detection rate of HAE candidates suggests an overdensity factor of δ_gal = 5.6 ± 0.3 and 4.9 ± 0.3 over the volume of 54 x 32 x 32 co-moving Mpc³. The overdensity factor increases two to three times when focusing on the high-density regions of scales 10-15 co-moving Mpc. Interestingly, the HAE density maps reveal that BOSS1244 contains a dominant structure, while BOSS1542 manifests as a giant filamentary structure. We measure the Hα luminosity functions (HLFs), finding that BOSS1244's HLF is nearly identical to that of the general field at the same epoch, while BOSS1542 shows an excess of HAEs with high Hα luminosity, indicating the presence of enhanced star formation or active galactic nuclei activity. We conclude that the two massive MAMMOTH overdensities are undergoing a rapid galaxy mass assembly.