2000AJ....119.1526S

We report on two programs to address differential evolution between the radio-loud and radio-quiet quasar populations at high (z>4) redshift. Both programs entail studying the radio properties of optically selected quasars. First, we have observed 32 optically selected, high-redshift (z>4) quasars with the VLA at 6 cm (5 GHz). These sources represent a statistically complete and well-understood sample. We detect four quasars above our 3 σ limit of ~0.15 mJy, which is sufficiently sensitive to detect all radio-loud quasars at the probed redshift range. Second, we have correlated 134 z>4 quasars, comprising all such sources that we are aware of as of mid-1999, with the FIRST survey and the NRAO VLA Sky Survey. These two recent 1.4 GHz VLA sky surveys reach 3 σ limits of approximately 0.6 and 1.4 mJy, respectively. We identify a total of 15 z>4 quasars, of which six were not previously known to be radio-loud. The depth of these surveys does not reach the radio-loud/radio-quiet demarcation luminosity density (L_1.4GHz=10^32.5 h^–2₅₀ ergs.s^–1.Hz^–1) at the redshift range considered; this correlation therefore provides only a lower limit to the radio-loud fraction of quasars at high redshift. The two programs together identify eight new radio-loud quasars at z>4, a significant increase over the seven currently in the published literature. We find no evidence that the radio-loud fraction depends on optical luminosity for -25>M_B>-28 at z≃2 or for -26>M_B>-28 at z>4. Our results also show no evolution in the radio-loud fraction between z≃2 and z>4 (for -26>M_B>-28).