PC 1158 + 4635 - an optically selected quasar with a redshift of 4.73

In this paper we present photometric and spectroscopic observations of PC 1158+4635, a quasar with a redshift of 4.73. This object was discovered by the detection of its Lyα emission line in a low-resolution CCD grism survey. The apparent magnitude of the quasar is r = 20.2; its absolute magnitude is similar to that of 3C 273 (for q_0_=0.5). The spectrum of PC 1158 + 4635 reveals prominent emission lines of Lyα and C IV and a sharp, approximately one magnitude drop in the continuum level across the Lyα emission line (features seen in most z > 4 quasars). The redshift-continuum depression relation for quasars requires a dramatic change in the "Lyman α clouds" (either in number or intrinsic properties) at a redshift of ~3.5 or else the presence of a new source that is responsible for the majority of the absorption at redshifts above four.