THE DISCOVERY OF A LARGE Lyα plus He II NEBULA AT z ≈ 1.67: A CANDIDATE LOW METALLICITY REGION?

We have discovered a approximate to 45 kpc Ly alpha nebula (or Ly alpha blob) at z approximate to 1.67 which exhibits strong, spatially extended He II emission and very weak C IV and C III] emission. This is the first spatially extended Ly alpha+He II emitter observed and the lowest redshift Ly alpha blob yet found. Strong Ly alpha and He II lambda 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 Ly alpha+He II emitters have surfaced either in Ly alpha-emitting galaxy surveys at high redshifts (z > 4) or in studies of Ly alpha 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 less than or similar to 10(-2)-10(-3) Z(circle dot)), but that this conclusion depends on the unknown ionization parameter of the system. The large He II equivalent width (approximate to 37 +/- 10 angstrom) and the large He II/Ly alpha ratio (0.12 +/- 0.04) suggest that the cloud is being illuminated by a hard ionizing continuum, either an active galactic nucleus (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 Ly alpha+He II 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 Ly alpha+He II signature as a unique tracer of primordial galaxy formation.