2021ApJ...922..236L

The MAMMOTH-1 nebula at z = 2.317 is an enormous Lyα nebula (ELAN) extending to a ∼440 kpc scale at the center of the extreme galaxy overdensity BOSS 1441. In this paper, we present observations of the CO(3 - 2) and 250 GHz dust-continuum emission from MAMMOTH-1 using the IRAM NOrthern Extended Millimeter Array. Our observations show that CO(3 - 2) emission in this ELAN has not extended widespread emission into the circum- and inter-galactic media. We also find a remarkable concentration of six massive galaxies in CO(3 - 2) emission in the central ∼100 kpc region of the ELAN. Their velocity dispersions suggest a total halo mass of M_200c ∼ 10^13.1M_☉, marking a possible protocluster core associated with the ELAN. The peak position of the CO(3 - 2) line emission from the obscured AGN is consistent with the location of the intensity peak of MAMMOTH-1 in the rest-frame UV band. Its luminosity line ratio between the CO(3 - 2) and CO(1 - 0)r_3,1 is 0.61 ± 0.17. The other five galaxies have CO(3 - 2) luminosities in the range of (2.1-7.1) x 10⁹ K km s^–1 pc², with the star-formation rates derived from the 250 GHz continuum of (<36)-224 M_☉ yr^–1. Follow-up spectroscopic observations will further confirm more member galaxies and improve the accuracy of the halo mass estimation.