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 similar or equal to 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(10) (L-bol/erg s(-1)) < 47.2. We employ ultraviolet spectral reconstructions based on an independent component analysis to parametrize the CIV.1549-emission line that is used to infer the strength of accretion disc winds, and the He II.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 He II equivalent widths as well as a Baldwin effect with bolometric luminosity, whilst the radio-loud population has mostly strong He II, consistent with a stronger soft X-ray flux. The presence of strong He II 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 time-scales.

Automated summary

By investigating the low-frequency radio and ultraviolet properties of a sample of around 10,500 quasars, it was found that the radio detection rate increases with blueshift while the radio-loud fraction decreases. The radio-quiet population shows a range of He II equivalent widths, while the radio-loud population mostly exhibits strong He II. The presence of similar ultraviolet properties but differing radio properties among quasars suggests that radio and ultraviolet emission may be tracing activity occurring on different time-scales.