LoTSS/HETDEX: Optical quasars I. Low-frequency radio properties of
  optically selected quasars
  The radio-loud/radio-quiet (RL/RQ) dichotomy in quasars is still an open
question. Although it is thought that accretion onto supermassive black holes
in the centre the host galaxies of quasars is responsible for some radio
continuum emission, there is still a debate as to whether star formation or
active galactic nuclei (AGN) activity dominate the radio continuum luminosity.
To date, radio emission in quasars has been investigated almost exclusively
using high-frequency observations in which the Doppler boosting might have an
important effect on the measured radio luminosity, whereas extended structures,
best observed at low radio frequencies, are not affected by the Doppler
enhancement. We used a sample of quasars selected by their optical spectra in
conjunction with sensitive and high-resolution low-frequency radio data
provided by the LOw Frequency ARray (LOFAR) as part of the LOFAR Two-Metre Sky
Survey (LoTSS) to investigate their radio properties using the radio loudness
parameter ($\mathcal{R} = \frac{L_{\mathrm{144-MHz}}}{L_{\mathrm{i\,band}}}$).
The examination of the radio continuum emission and RL/RQ dichotomy in quasars
exhibits that quasars show a wide continuum of radio properties (i.e. no clear
bimodality in the distribution of $\mathcal{R}$). Radio continuum emission at
low frequencies in low-luminosity quasars is consistent with being dominated by
star formation. We see a significant albeit weak dependency of $\mathcal{R}$ on
the source nuclear parameters. For the first time, we are able to resolve radio
morphologies of a considerable number of quasars. All these crucial results
highlight the impact of the deep and high-resolution low-frequency radio
surveys that foreshadow the compelling science cases for the Square Kilometre
Array (SKA).
