We have discovered a 300 kpc-wide giant Lyman- α (Ly α) nebula centered on the massive galaxy group RO-1001 at z = 2.91 in the Cosmic Evolution Survey field. Keck Cosmic Web Imager observations reveal three cold gas filaments converging into the center of the potential well of its ∼4 × 10 13M⊙ dark matter halo, hosting 1200 M⊙ yr −1 of star formation as probed by Atacama Large Millimeter Array and NOrthern Extended Millimeter Array observations. The nebula morphological and kinematics properties and the prevalence of blueshifted components in the Ly α spectra are consistent with a scenario of gas accretion. The upper limits on active galactic nuclei activity and overall energetics favor gravity as the primary Ly α powering source and infall as the main source of gas flows to the system. Although interpretational difficulties remain, with outflows and likely also photoionization with ensuing recombination still playing a role, this finding provides arguably an ideal environment to quantitatively test models of cold gas accretion and galaxy feeding inside an actively star-forming massive halo at high redshift.
