Three Lyman-alpha emitting filaments converging to a massive galaxy
  group at z=2.91: discussing the case for cold gas infall
  We have discovered a 300kpc-wide giant Lya nebula centered on the massive
galaxy group RO-1001 at z=2.91 in the COSMOS field. Keck Cosmic Web Imager
observations reveal three cold gas filaments converging into the center of the
potential well of its ~4x10^13Msun dark matter halo, hosting 1200Msun/yr of
star formation as probed by ALMA and NOEMA observations. The nebula
morphological and kinematics properties and the prevalence of blueshifted
components in the Lya spectra are consistent with a scenario of gas accretion.
The upper limits on AGN activity and overall energetics favor gravity as the
primary Lya 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.
