Differential evolution of radio quiet and radio loud quasars

We have observed 40 optically selected high-redshift quasars with the VLA at 1515 MHz (20 cm). Three quasars are detected above our 3σ limit of 0.2 mJy. We review all available evidence on radio detections of optically selected quasars in terms of their ratio R of radio to optical luminosity. We find that the quasars ranked in the top 5-20 percent in R show strong evolution in their R distribution. We present two interpretations of this phenomenon. The first one poses that the R values decrease by a factor of around 100 from redshift 0.3 to 1.3. In the second interpretation, we consider that there are two populations of quasars, which we call radio silent and radio active, separated by R~0.1. The radio-silent fraction of quasars increases from ~16% at redshift 0.3, to ~75% at redshift 1.3.