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We studied a sample of 274 radio and X-ray selected quasars (XQSOs) detectedin the COSMOS and XXL-S radio surveys at 3 GHz and 2.1 GHz, respectively. Thissample was identified by adopting a conservative threshold in X-ray luminosity,Lx [2-10\ keV] >= 10^44 erg/s, selecting only the most powerful quasars. Usingavailable multiwavelength data, we examined various criteria for the selectionof radio-loud (RL) and radio-quiet (RQ) XQSOs, finding that the number of RL/RQXQSOs changes significantly depending on the chosen criterion. This discrepancyarises due to the different criteria tracing different physical processes anddue to our sample being selected from flux-limited radio and X-ray surveys.Another approach to study the origin of radio emission in XQSOs is via theirradio luminosity function (RLF). We constructed the XQSO 1.4 GHz radioluminosity functions (RLFs) in six redshift bins at 0.5 <= z <= 3.7. Thelower-1.4 GHz luminosity end shows a higher normalization than expected onlyfrom AGN contribution in all studied redshift bins. The found "bump" is mostlydominated by emission due to star-forming (SF) processes within the XQSO hostgalaxies. As expected, AGN-related radio emission dominates at thehigher-luminosity end of RLF. The evolution of XQSO RLF was studied viacombination of analytic forms from the literature to constrain thelower-luminosity "bump" and the higher-luminosity AGN part of the RLF. Wedefined two 1.4 GHz luminosity thresholds, L_th,SF and L_th,AGN, below andabove which more than 80% of sources contributing to the RLF are dominated bySF and AGN-related activity, respectively. These thresholds evolve withredshift, most likely due to the strong evolution of SFRs of the XQSO hostgalaxies.

astronomy and astrophysics

The XXL Survey: XLIII. The quasar radio loudness dichotomy exposed via  radio luminosity functions obtained by combining results from COSMOS and  XXL-S X-ray selected quasars