THE RADIO PROPERTIES OF OPTICALLY SELECTED QUASARS .3. COMPARISON BETWEEN OPTICAL AND X-RAY SELECTED SAMPLES

A sample of 103 quasars from the Large Bright Quasar Survey (LBQS) has been observed with the VLA at 8.4 GHz to study the evolution of the r adio luminosity distribution and its dependence on absolute magnitude. Radio data from pointed observations are now available for 359 of the 1055 LBQS quasars. The radio-loud fraction is constant at approximate to 10% over the absolute magnitude range -28 less than or similar to M(B) less than or similar to -23, and it rises to similar to 20% (log R(8.4) > 1) or similar to 35% (log L(8.4) > 25) at the brightest absol ute magnitudes in the sample. This nearly flat distribution differs ma rkedly from those of the optically selected Palomar-Green (PG) Bright Quasar Survey and the X-ray selected Extended Medium Sensitivity Surve y (EMSS), both of which have lower radio-loud fractions for absolute m agnitudes fainter than M(B) = -24 and higher fractions at brighter mag nitudes. The reason for the high radio-loud fraction at bright absolut e magnitudes in the PG, compared to the LBQS and other optically selec ted quasar surveys, is unknown. The trend of increasing radio-loud fra ction with absolute magnitude in the EMSS is due at least in part to a correlation between X-ray and radio luminosity. Combining the LBQS da ta with radio studies of high-redshift quasars leads to the conclusion that the radio-loud fraction in optically selected quasars does not a ppear to evolve significantly, aside from a modest increase at z simil ar to 1, from z = 0.2 to redshifts approaching 5, a result contrary to previous studies that found a decrease in radio-loud fraction with in creasing redshift by comparing the low-z fraction in the PG to higher redshift samples.