The Radio Properties of Optically Selected Quasars. III. Comparison between Optical and X‐Ray Selected Samples

A sample of 103 quasars from the Large Bright Quasar Survey (LBQS) has beenobserved with the VLA at 8.4 GHz to study the evolution of the radio luminositydistribution and its dependence on absolute magnitude. Radio data from pointedobservations are now available for 359 of the 1055 LBQS quasars. The radio-loudfraction is constant at ~10% over the absolute magnitude range -28 <= MB <=-23, and it rises to ~20% (log R > 1) or ~35% (log L > 25) at the brightestabsolute magnitudes in the sample. This nearly flat distribution differsmarkedly from those of the optically selected Palomar-Green (PG) Bright QuasarSurvey and the X-ray selected Extended Medium Sensitivity Survey (EMSS), bothof which have lower radio-loud fractions for absolute magnitudes fainter thanMB = -24 and higher fractions at brighter magnitudes. The reason for the highradio-loud fraction at bright absolute magnitudes in the PG, compared to theLBQS and other optically selected quasar surveys, is unknown. The trend ofincreasing radio-loud fraction with absolute magnitude in the EMSS is due atleast in part to a correlation between X-ray and radio luminosity. Combiningthe LBQS data with radio studies of high-redshift quasars leads to theconclusion that the radio-loud fraction in optically selected quasars does notappear to evolve significantly, aside from a modest increase at z ~1, from z =0.2 to redshifts approaching 5, a result that is contrary to previous studieswhich found a decrease in radio-loud fraction with increasing redshift bycomparing the low-z fraction in the PG to higher redshift samples.Comment: 25 page manuscript and 2 tables in AASTeX Latex (aaspp4.sty and apjpt4.sty); 7 postscript figures...accepted for publication in the Astrophysical Journa