Radio emission from radio-quiet quasars

Abstract

In a search for radio emission from bright, optically selected quasars, we detected ∼50% of the quasars at 5 GHz. This radio detection rate is so much higher than that for faint quasars (<10%)1–4, that it requires that the distribution of radio(R)-to-optical(O) luminosity ratios (LR/LO) be correlated with optical properties and/or with distance (or cosmic epoch). The values of LR/LO in our sample range from ∼10−2 to <10−5 (where radio detection is no longer possible). So broad and smooth a distribution argues against a distinct dichotomy between ‘radio-quiet’ and ‘radio-loud’ quasars. The quasars in our sample which remain undetected at radio frequencies must exhibit a spectral-flux distribution rising from the radio to optical, previously considered an ‘abnormal’ spectrum for a quasar.