We report observations of a luminous unresolved object at redshift z = 4.62, with a featureless optical spectrum redward of the Lya forest region, discovered from Sloan Digital Sky Survey commissioning data. The redshift is determined by the onset of the Lya forest at Ao and a Lyman limit system at Ao l ∼ 6800 l = 5120. 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 226.6 at 1450 Ao in the rest frame (h = 0.5, ;= 0.5) and therefore 0 0 cannot be an ordinary galaxy. It shows no radio emission (the 3 j upper limit of its flux at 6 cm is 60 mJy), 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 j 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.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science