The cold circumgalactic environment of MAMMOTH-I: dynamically cold gas
  in the core of an Enormous Ly-alpha Nebula
  The MAMMOTH-I Nebula at redshift 2.3 is one of the largest known Ly-alpha
nebulae in the Universe, spanning ~440 kpc. Enormous Ly-alpha nebulae like
MAMMOTH-I typically trace the densest and most active regions of galaxy
formation. Using sensitive low-surface-brightness observations of CO(1-0) with
the Very Large Array, we trace the cold molecular gas in the inner 150 kpc of
the MAMMOTH-I Nebula. CO is found in four regions that are associated with
either galaxies or groups of galaxies that lie inside the nebula. In three of
the regions, the CO stretches up to ~30 kpc into the circum-galactic medium
(CGM). In the centermost region, the CO has a very low velocity dispersion
(FWHM$_{\rm CO}$ ~ 85 km/s), indicating that this gas is dynamically cold. This
dynamically cold gas coincides with diffuse restframe optical light in the CGM
around a central group of galaxies, as discovered with the Hubble Space
Telescope. We argue that this likely represents cooling of settled and enriched
gas in the center of MAMMOTH-I. This implies that the dynamically cold gas in
the CGM, rather than the obscured AGN, marks the core of the potential well of
this Ly-alpha nebula. In total, the CO in the MAMMOTH-I Nebula traces a
molecular gas mass of M$_{\rm H2}$ ~ 1.4 ($\alpha_{\rm CO}$/3.6) $\times$
10$^{11}$ M$_{\odot}$, with roughly 50% of the CO(1-0) emission found in the
CGM. Our results add to the increasing evidence that extended reservoirs of
molecular gas exist in the CGM of massive high-z galaxies and proto-clusters.
