Placing LOFAR-detected quasars in CIV emission space: implications for
  winds, jets and star formation
  We present an investigation of the low-frequency radio and ultraviolet
properties of a sample of $\simeq$10,500 quasars from the Sloan Digital Sky
Survey Data Release 14, observed as part of the first data release of the
Low-Frequency-Array Two-metre Sky Survey. The quasars have redshifts $1.5 < z <
3.5$ and luminosities $44.6 < \log(L_{\text{bol}}/\text{erg s}^{-1}) < 47.2$.
We employ ultraviolet spectral reconstructions based on an independent
component analysis to parametrize the CIV$\lambda$1549-emission line that is
used to infer the strength of accretion disc winds, and the HeII$\lambda$1640
line, an indicator of the soft X-ray flux. We find that radio-detected quasars
are found in the same region of CIV blueshift versus equivalent-width space as
radio-undetected quasars, but that the loudest, most luminous and largest radio
sources exist preferentially at low CIV blueshifts. Additionally, the
radio-detection fraction increases with blueshift whereas the radio-loud
fraction decreases. In the radio-quiet population, we observe a range of HeII
equivalent widths as well as a Baldwin effect with bolometric luminosity,
whilst the radio-loud population has mostly strong HeII, consistent with a
stronger soft X-ray flux. The presence of strong HeII is a necessary but not
sufficient condition to detect radio-loud emission suggesting some degree of
stochasticity in jet formation. Using energetic arguments and Monte Carlo
simulations, we explore the plausibility of winds, compact jets and star
formation as sources of the radio quiet emission, ruling out none. The
existence of quasars with similar ultraviolet properties but differing radio
properties suggests, perhaps, that the radio and ultraviolet emission is
tracing activity occurring on different timescales.
