Abstract
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 inwards
deceleration of the filament and the prevalence of blueshifted components in
the Lya spectra support 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
recombinations still playing a role, this finding provides arguably an ideal
environment to quantitatively test models of cold gas accretion and galaxy
feeding inside a massive halo at high redshift.
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