We have discovered a \approx 45 kpc Ly \alpha nebula ( or Ly \alpha “ blob ” ) at z \approx 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 \lesssim 10 ^ { -2 } -10 ^ { -3 } Z _ { \sun } ) , but that this conclusion depends on the unknown ionization parameter of the system .
The large He ii equivalent width ( \approx 37 \pm 10Å ) and the large He ii /Ly \alpha ratio ( 0.12 \pm 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 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 .