Discovery of a protocluster core associated with an enormous Ly$\alpha$
  Nebula at $z = 2.3$
  The MAMMOTH-1 nebula at $z=2.317$ is an enormous Ly$\alpha$ nebula (ELAN)
extending to a $\sim$440 kpc scale at the center of the extreme galaxy
overdensity BOSS 1441. In this paper, we present observations of the $\rm
CO(3-2)$ and 250 GHz dust-continuum emission from the MAMMOTH-1 using the IRAM
NOrthern Extended Millimeter Array. Our observations show that $\rm CO(3-2)$
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 $\rm CO(3-2)$ emission in the central $\sim$100 kpc region of the
ELAN. Their velocity dispersions suggest a total halo mass of $M_{200c} \sim
10^{13.1} M_{\odot}$, marking a possible protocluster core associated with the
ELAN. The peak position of the $\rm CO(3-2)$ 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 $\rm CO(3-2)$ and
$\rm CO(1-0)$ $r_{3,1}$ is 0.61$\pm$0.17. The other five galaxies have $\rm
CO(3-2)$ luminosities in the range of (2.1-7.1)$\times 10^9$ K $\rm km\,s^{-1}$
pc$^2$, with the star-formation rates derived from the 250GHz continuum of
($<$36)-224 $M_{\odot}$yr$^{-1}$. Follow-up spectroscopic observations will
further confirm more member galaxies and improve the accuracy of the halo mass
estimation.
