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We have discovered a ~45 kpc Lya nebula (or Lya ``blob'') at z~1.67 whichexhibits strong, spatially-extended HeII emission and very weak CIV and CIII]emission. This is the first spatially-extended Lya+HeII emitter observed andthe lowest redshift Lya blob yet found. Strong Lya and HeII-1640 emission inthe absence of metal lines has been proposed as a unique observationalsignature of primordial galaxy formation (e.g., from gravitational coolingradiation or Population III star formation), but no convincing examples ofspatially-extended Lya+HeII emitters have surfaced either in Lya-emittinggalaxy surveys at high redshifts (z > 4) or in studies of Lya nebulae at lowerredshifts. From comparisons with photoionization models, we find that theobserved 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 unknownionization 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 beingilluminated by a hard ionizing continuum, either an AGN or very low metallicitystars, or perhaps powered by gravitational cooling radiation. Thus far there isno obvious sign of a powerful AGN in or near the system, so in order to powerthe nebula while remaining hidden from view even in the mid-infrared, the AGNwould need to be heavily obscured. Despite the strong Lya+HeII emission, it isnot yet clear what is the dominant power source for this nebula. The systemtherefore serves as an instructive example of how the complexities of trueastrophysical sources will complicate matters when attempting to use a strongLya+HeII signature as a unique tracer of primordial galaxy formation.

The Discovery of a Large Lyman-alpha+HeII Nebula at z~1.67: A Candidate Low Metallicity Region?