Radio Properties of [CLC][ITAL]z[/ITAL][/CLC] > 4 Optically Selected Quasars

We report on two programs to address differential evolution between theradio-loud and radio-quiet quasar populations at high (z>4) redshift. Bothprograms entail studying the radio properties of optically-selected quasars.First, we have observed 32 optically-selected, high-redshift (z>4) quasars withthe VLA at 6 cm (5 GHz). These sources comprise a statistically complete andwell-understood sample. We detect four quasars above our 3-sigma limit of ~0.15mJy, which is sufficiently sensitive to detect all radio-loud quasars at theprobed redshift range. Second, we have correlated 134 z>4 quasars, comprisingall such sources that we are aware of as of mid-1999, with FIRST and NVSS.These two recent 1.4 GHz VLA sky surveys reach 3-sigma limits of approximately0.6 mJy and 1.4 mJy respectively. We identify a total of 15 z>4 quasars, ofwhich six were not previously known to be radio-loud. The depth of thesesurveys does not reach the radio-loud/radio-quiet demarcation luminositydensity (L(1.4 GHz) = 10^32.5 h(50)^(-2) ergs/s/Hz) at the redshift rangeconsidered; this correlation therefore only provides a lower limit to theradio-loud fraction of quasars at high-redshift. The two programs togetheridentify eight new radio-loud quasars at z>4, a significant increase over theseven currently in the published literature. We find no evidence for radio-loudfraction depending 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-loudfraction between z~2 and z>4 (-26>M_B>-28).Comment: 19 pages, 7 figures; to appear in The Astronomical Journal (April 2000