2019ApJ...887...86E

The MAMMOTH-I Nebula at redshift 2.3 is one of the largest known Lyα nebulae in the universe, spanning ∼440 kpc. Enormous Lyα 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 circumgalactic medium (CGM). In the centermost region, the CO has a very low velocity dispersion (FWHM_CO ∼ 85 km s^–1), indicating that this gas is dynamically cold. This dynamically cold gas coincides with diffuse rest-frame 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 active galactic nucleus, marks the core of the potential well of this Lyα nebula. In total, the CO in the MAMMOTH-I Nebula traces a molecular gas mass of M_H2 ∼ 1.4(α_CO/3.6) x 10¹¹ M_☉, 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 protoclusters.