Extended and filamentary Lyα emission from the formation of a protogalactic halo at z=2.63

We report the observation of a further asymmetric, extended Ly alpha emitting halo at z = 2.63, from our ultra-deep, long-slit spectroscopic survey of faint high-redshift emitters, undertaken with Magellan LDSS3 in the GOODS-S field. The Ly alpha emission, detected over more than 30 kpc, is spatially coincident with a statistically significant concentration of galaxies visible in deep broad-band imaging. While these faint galaxies without spectroscopic redshifts cannot all with certainty be associated with one another or with the Ly alpha emission, there are a number of compelling reasons why they very probably form a Milky Way halo-mass group at the Ly alpha redshift. Filamentary structure, possibly consisting of Ly alpha emission with very high equivalent width, blue stellar continua and evidence for disturbed stellar populations, suggest that the properties of the emitting region reflect ongoing galaxy assembly, with recent galaxy mergers and star formation occurring in the group. The Ly alpha emission may be powered by cooling radiation or spatially extended star formation in the halo, but is unlikely to be fluorescence driven by either an active galactic nucleus or one of the galaxies. A comparison with the Ly alpha surface brightness profiles of more typical, bright Ly alpha emitters or Lyman break galaxies from similarly deep two-dimensional spectra shows them to be conspicuously different from the extended, asymmetric object studied here. This is consistent with the picture that typical Ly alpha emitters represent Ly alpha resonantly scattering from single, kinematically quiescent, compact sources of ionizing radiation, whereas extended emission of the kind seen in the current halo reflect a more active, kinematically disturbed stage in the galaxy formation process. Hence, unusual Ly alpha emission as observed here may provide unique insights into what is probably a key mode of galaxy formation at high redshifts. Our observations provide further, circumstantial evidence that galaxy mergers may promote the production and/or escape of ionizing radiation, and that the haloes of interacting galaxies may be significant sources for ionizing photons during the epoch of reionization.