The discovery of a high-redshift quasar without emission lines from Sloan Digital Sky Survey commissioning data

We report observations of a luminous unresolved object at redshift z = 4.62, with a featureless optical spectrum redward of the Lyα forest region, discovered from Sloan Digital Sky Survey commissioning data. The redshift is determined by the onset of the Lyα forest at λ ∼ 6800 Å and a Lyman limit system at λ = 5120 Å. A strong Lya absorption system with weak metal absorption lines at z = 4.58 is also identified in the spectrum. The object has a continuum absolute magnitude of -26.6 at 1450 Å in the rest frame (h₀ = 0.5, q₀ = 0.5) and therefore cannot be an ordinary galaxy. It shows no radio emission (the 3 σ upper limit of its flux at 6 cm is 60 μJy), indicating a radio-to-optical flux ratio at least as small as that of the radio-weakest BL Lacertae objects known. It is also not linearly polarized to a 3 σ upper limit of 4% in the observed I band. Therefore, it is either the most distant BL Lac object known to date, with very weak radio emission, or a new type of unbeamed quasar, whose broad emission line region is very weak or absent.