Extremely loud & incredibly far: observing radio bright AGN into the cosmic dawn

In this thesis new methodologies are developed for the detection and systematic study of radio sources in the early universe. This allows us to gain a deeper understanding of the formation and evolution of galaxies, the activity of supermassive black holes, and the final phase transition of our universe: the epoch of reionization. Using the Low Frequency Array (LOFAR) telescope, this thesis systematically investigates the low radio frequency properties of quasars, the brightest non-variable objects in our cosmos, in the first billion years after the Big Bang. Through the discovery of new radio quasars in the early universe and subsequent studies, this thesis shows the diversity within the quasar population and highlights the importance of multi-wavelength observations for our comprehension of the formation and evolution of active supermassive black holes and their impact on the surrounding environment.Large scale structure and cosmolog

Discovery of 24 radio-bright quasars at 4.9 <= z <= 6.6 using low-frequency radio observations

Galaxie

Discovery of 24 radio-bright quasars at 4.9 <= z <= 6.6 using low-frequency radio observations

Galaxie

V-LoTSS: The circularly polarised LOFAR Two-metre Sky Survey

We present the detection of 68 sources from the most sensitive radio survey in circular polarisation conducted to date. We used the second data release of the 144 MHz LOFAR Two-metre Sky Survey to produce circularly polarised maps with a median noise of 140 µJy beam−1 and resolution of 20″ for ≈27% of the northern sky (5634 deg2). The leakage of total intensity into circular polarisation is measured to be ≈0.06%, and our survey is complete at flux densities ≥1 mJy. A detection is considered reliable when the circularly polarised fraction exceeds 1%. We find the population of circularly polarised sources is composed of four distinct classes: stellar systems, pulsars, active galactic nuclei, and sources unidentified in the literature. The stellar systems can be further separated into chromospherically active stars, M dwarfs, and brown dwarfs. Based on the circularly polarised fraction and lack of an optical counterpart, we show it is possible to infer whether the unidentified sources are likely unknown pulsars or brown dwarfs. By the completion of this survey of the northern sky, we expect to detect 300±100 circularly polarised sources