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 Ly alpha forest region , discovered from Sloan Digital Sky Survey commissioning data. The redshift is determined by the onset of the Ly alpha forest at lambda similar to 680 0 Angstrom and a Lyman limit system at lambda = 5120 Angstrom. A strong Ly alpha absorption system with weak metal absorption lines at z = 4.58 is als o identified in the spectrum. The object has a continuum absolute magnitude of -26.6 at 1450 Angstrom in the rest frame (h(0) = 0.5, q(0) = 0.5) and t herefore cannot be an ordinary galaxy. It shows no radio emission (the 3 si gma upper limit of its flux at 6 cm is 60 mu Jy), indicating a radio-to-opt ical 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 sigma upper limit o f 4% in the observed I band. Therefore, it is either the most distant BL La c object known to date, with very weak radio emission, or a new type of unb eamed quasar, whose broad emission line region is very weak or absent.