Extended and Filamentary Lyman Alpha Emission from the Formation of a
  Protogalactic Halo at z=2.63
  We report the observation of a further asymmetric, extended Lyman 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 Lya emission, detected over more than 30 kpc, is spatially
coincident with a concentration of galaxies visible in deep broad-band imaging.
While these faint galaxies without spectroscopic redshifts cannot with
certainty be associated with one another or with the Lya emission, there are a
number of compelling reasons why they very probably form a Milky Way halo-mass
group at the Lya redshift. A filamentary structure, possibly consisting of Lya
emission at very high equivalent width, 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. Hence, the Lya provides unique insights into what is probably a key mode
of galaxy formation at high redshifts. The Lya 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 AGN or one of the galaxies. The
spatial profile of the emission is conspicuously different from that of typical
Lya emitters or Lyman break galaxies, which is consistent with the picture that
extended emission of this kind represents a different stage in the galaxy
formation process. Faint, extended Lya emitters such as these may be lower-mass
analogues of the brighter Lya blobs. Our observations provide further,
circumstantial evidence that galaxy mergers may promote the production and / or
escape of ionizing radiation, and that the halos of interacting galaxies may be
significant sources for ionizing photons during the epoch of reionization.
