The Discovery of a Large Lyman-alpha+HeII Nebula at z~1.67: A Candidate
  Low Metallicity Region?
  We have discovered a ~45 kpc Lya nebula (or Lya ``blob'') at z~1.67 which
exhibits strong, spatially-extended HeII emission and very weak CIV and CIII]
emission. This is the first spatially-extended Lya+HeII emitter observed and
the lowest redshift Lya blob yet found. Strong Lya and HeII-1640 emission in
the absence of metal lines has been proposed as a unique observational
signature of primordial galaxy formation (e.g., from gravitational cooling
radiation or Population III star formation), but no convincing examples of
spatially-extended Lya+HeII emitters have surfaced either in Lya-emitting
galaxy surveys at high redshifts (z > 4) or in studies of Lya nebulae at lower
redshifts. From comparisons with photoionization models, we find that the
observed line ratios in this nebula are consistent with low metallicity gas (Z
< 10^-2 - 10^-3 Z_sun), but that this conclusion depends on the unknown
ionization parameter of the system. The large HeII equivalent width (~37+/-10A)
and the large HeII/Lya ratio (0.12+/-0.04) suggest that the cloud is being
illuminated by a hard ionizing continuum, either an AGN or very low metallicity
stars, or perhaps powered by gravitational cooling radiation. Thus far there is
no obvious sign of a powerful AGN in or near the system, so in order to power
the nebula while remaining hidden from view even in the mid-infrared, the AGN
would need to be heavily obscured. Despite the strong Lya+HeII emission, it is
not yet clear what is the dominant power source for this nebula. The system
therefore serves as an instructive example of how the complexities of true
astrophysical sources will complicate matters when attempting to use a strong
Lya+HeII signature as a unique tracer of primordial galaxy formation.
