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 Surveycommissioning data. The redshift is determined by the onset of the Lyα forest at λ ~ 6800 A and a Lyman limitsystem at λ = 5120 A. A strong Lyα absorption system with weak metal absorption lines at z = 4.58 is also identified in the spectrum. The object has a continuum absolute magnitudeof -26.6 at 1450 A 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 Lacertaeobjects 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 objectknown to date, with very weak radio emission, or a new type of unbeamed quasar, whose broad emission line region is very weak or absent.