The radio loudness of SDSS quasars from the LOFAR Two-metre Sky Survey:
  ubiquitous jet activity and constraints on star formation
  We examine the distribution of radio emission from ~42,000 quasars from the
Sloan Digital Sky Survey, as measured in the LOFAR Two-Metre Sky Survey
(LoTSS). We present a model of the radio luminosity distribution of the quasars
that assumes that every quasar displays a superposition of two sources of radio
emission: active galactic nuclei (jets) and star-formation. Our two-component
model provides an excellent match to the observed radio flux density
distributions across a wide range of redshifts and quasar optical luminosities;
this suggests that the jet-launching mechanism operates in all quasars but with
different powering efficiency. The wide distribution of jet powers allows for a
smooth transition between the 'radio-quiet' and 'radio-loud' quasar regimes,
without need for any explicit bimodality. The best-fit model parameters
indicate that the star-formation rate of quasar host galaxies correlates
strongly with quasar luminosity and also increases with redshift at least out
to z~2. For a model where star-formation rate scales as $SFR \propto
L_{bol}^\alpha (1+z)^\beta$, we find $\alpha = 0.47 \pm 0.01$ and $\beta = 1.61
\pm 0.05$, in agreement with far-infrared studies. Quasars contribute ~0.15 per
cent of the cosmic star-formation rate density at z=0.5, rising to 0.4 per cent
by z=2. The typical radio jet power is seen to increase with both increasing
optical luminosity and black hole mass independently, but does not vary with
redshift, suggesting intrinsic properties govern the production of the radio
jets. We discuss the implications of these results for the triggering of quasar
activity and the launching of jets.
