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
  We studied a sample of 274 radio and X-ray selected quasars (XQSOs) detected
in the COSMOS and XXL-S radio surveys at 3 GHz and 2.1 GHz, respectively. This
sample 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. Using
available multiwavelength data, we examined various criteria for the selection
of radio-loud (RL) and radio-quiet (RQ) XQSOs, finding that the number of RL/RQ
XQSOs changes significantly depending on the chosen criterion. This discrepancy
arises due to the different criteria tracing different physical processes and
due 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 their
radio luminosity function (RLF). We constructed the XQSO 1.4 GHz radio
luminosity functions (RLFs) in six redshift bins at 0.5 <= z <= 3.7. The
lower-1.4 GHz luminosity end shows a higher normalization than expected only
from AGN contribution in all studied redshift bins. The found "bump" is mostly
dominated by emission due to star-forming (SF) processes within the XQSO host
galaxies. As expected, AGN-related radio emission dominates at the
higher-luminosity end of RLF. The evolution of XQSO RLF was studied via
combination of analytic forms from the literature to constrain the
lower-luminosity "bump" and the higher-luminosity AGN part of the RLF. We
defined two 1.4 GHz luminosity thresholds, L_th,SF and L_th,AGN, below and
above which more than 80% of sources contributing to the RLF are dominated by
SF and AGN-related activity, respectively. These thresholds evolve with
redshift, most likely due to the strong evolution of SFRs of the XQSO host
galaxies.
