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 Lyman alpha forest region, discovered from Sloan Digital Sky Survey (SDSS) commissioning data. The redshift is determined by the onset of the Lyman-alpha forest at lambda ~ 6800 A, and a Lyman Limit System at lambda = 5120 A. A strong Ly_alpha 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 A in the rest-frame (h_0=0.5, q_0=0.5), and therefore cannot be an ordinary galaxy. It shows no radio emission (the 3-sigma upper limit of its flux at 6 cm is 60 micron Jy), indicating an radio-to-optical flux ratio at least as small as that of the radio-weakest known BL Lacs. It is also not linearly polarized in the observed I band to a 3-sigma upper limit of 4%. 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.