The radio-loud fraction of quasars is a strong function of redshift and optical luminosity

Using a sample of optically selected quasars from the Sloan Digital Sky Survey, we have determined the radio-loud fraction (RLF) of quasars as a function of redshift and optical luminosity. The sample contains more than 30,000 objects and spans a redshift range of 0 < z <= 5 and a luminosity range of -30 <= Mi < -22. We use both the radio-to-optical flux ratio (R parameter) and the radio luminosity to define radio- loud quasars. After breaking the correlation between redshift and luminosity due to the flux-limited nature of the sample, we find that the RLF of quasars decreases with increasing redshift and decreasing luminosity. The relation can be described in the form of log [RLF/(1-RLF)] = b(0) + b(z) log (1 + z) + b(M)(M-2500 + 26), where M-2500 is the absolute magnitude at rest-frame 2500 angstrom, and b(z), b(M) < 0. When using R > 10 to define radio- loud quasars, we find that b(0) = -0.132 +/- 0.116, b(z) = -2.052 +/- 0.261, and b(M) = -0.183 +/- 0.025. The RLF at z = 0.5 declines from 24.3% to 5.6% as luminosity decreases from M-2500 = -26 to -22, and the RLF at M-2500 = -26 declines from 24.3% to 4.1% as redshift increases from 0.5 to 3, suggesting that the RLF is a strong function of both redshift and luminosity. We also examine the impact of flux-related selection effects on the RLF determination using a series of tests and find that the dependence of the RLF on redshift and luminosity is highly likely to be physical, and that the selection effects we considered are not responsible for the dependence.