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

We have discovered a ≈45 kpc Lyα nebula (or Lyα “blob”) at z ≈ 1.67 which exhibits strong, spatially-extended He ii emission and very weak C iv and C iii] emission. This is the first spatiallyextended Lyα+He ii emitter observed and the lowest redshift Lyα blob yet found. Strong Lyα and He iiλ1640 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 Lyα+He ii emitters have surfaced either in Lyα-emitting galaxy surveys at high redshifts (z > 4) or in studies of Lyα 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 . 10−10Z⊙), but that this conclusion depends on the unknown ionization parameter of the system. The large He ii equivalent width (≈37±10Å) and the large He ii/Lyα 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 Lyα+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 Lyα+He ii signature as a unique tracer of primordial galaxy formation. Subject headings: galaxies: formation — galaxies: high-redshift — galaxies: surveys