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 SkySurvey, we have determined the radio-loud fraction (RLF) of quasars as afunction of redshift and optical luminosity. The sample contains more than30,000 objects and spans a redshift range of 010 todefine 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% asluminosity decreases from M_{2500}=-26 to M_{2500}=-22, and the RLF atM_{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 RLFdetermination using a series of tests, and find that the dependence of the RLFon redshift and luminosity is highly likely to be physical, and the selectioneffects we considered are not responsible for the dependence.Comment: 23 pages, 8 figures. Accepted for publication in Ap