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While radio emission in quasars can be contributed to by a variety ofprocesses (involving star forming regions, accretion disk coronas and winds,and jets), the powering of the radio loudest quasars must involve very strongjets, presumably launched by the Blandford-Znajek mechanism incorporating themagnetically arrested disk (MAD) scenario. We focus on the latter andinvestigate the dependence of their fraction on redshift. We also examine thedependence of the radio-loud fraction (RLF) on BH mass ($M_{\rm BH}$) andEddington ratio ($\lambda_{\rm Edd}$) while excluding the redshift bias bynarrowing its range. In both these investigations we remove the bias associatedwith: (1) the diversity of source selection by constructing two well-defined,homogeneous samples of quasars (first within $0.7 \leq z < 1.9$, second within$0.5 \leq z < 0.7$); (2) a strong drop in the RLF of quasars at smaller BHmasses by choosing those with BH masses larger than $10^{8.5} M_{\odot}$. Weconfirm some previous results showing the increase in the fraction ofradio-loud quasars with cosmic time and that this trend can be even steeper ifwe account for the bias introduced by the dependence of the RLF on BH masswhereas the bias introduced by the dependence of the RLF on Eddington ratio isshown to be negligible. Assuming that quasar activities are triggered by galaxymergers we argue that such an increase can result from the slower drop withcosmic time of mixed mergers than of wet mergers.

The dependence of the fraction of radio luminous quasars on redshift and its theoretical implications