Discovery of a Protocluster Core Associated with an Enormous Lya Nebula at z = 2.3

Abstract

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 ~ 10^13.1 M _⊙, 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) × 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.