Radio Emission from Broad Absorption Line Quasars: Origins and Implications

Abstract

A fraction of quasars display signatures of outflowing winds with velocities of up to in their UV/optical spectra. These Broad Absorption Line Quasars (BALQSOs) have been studied for over 40 years but a consensus understanding of their appearance is yet to emerge. In particular, BALQSOs have been known to exhibit different behaviour at radio wavelengths to the general population, a key feature that both plausible orientation or evolutionary models must be able to explain.

After providing the necessary background and motivation (Chapters 1 and 2), I build on the study of by utilising the second data release of the LOFAR Two Metre Sky Survey to study the largest sample of radio-detected BALQSOs ever assembled. We firmly place BALQSOs in the radio-quiet regime and demonstrate that they have a significant enhancement in radio emission as compared to non-BALQSOs at low frequencies. We use composite optical spectra of different populations to demonstrate that radio-detection is connected to features visible in the wind component of the spectra. Although we cannot determine the origin of radio emission in the vast majority of sources, this provides a clear connection between the radio and the wind itself (Chapter 3).


Following on, I engage with the latest research on radio-quiet quasars by folding in other recently discovered connections between quasar optical spectra and radio emission. Quasars with redder optical colours have been found to show a similar radio enhancement as BALQSOs which is explored deeply in the theses of Lizelke Klindt and Victoria Fawcett both also at Durham University . Also, quasars with stronger blueshifts as compared to their equivalent widths, also characterised as having a higher , show an enhancement in radio-detection fraction. We noted that BALQSOs preferentially occupy the red quasar population and have higher average distances and aimed to elucidate connections and differences between these features. We show that each of these optical features has its own connection to radio-detection fraction although colour seems to be the most dominant. We also apply the current standard for estimating radio emission from wind shocks and find that they can plausibly explain a substantial fraction of radio emission in radio-quiet quasars with only moderate efficiencies, suggesting advancement of these models is needed in order for them to become useful discriminatory tools (Chapter 5).

During the course of Chapter 3, I discovered differences in the relationship between radio-detected HiBALs and LoBALs, the two key subsets of BALQSOs. LoBALs showed more dramatic changes in their optical spectra when radio-detected as compared to HiBALs. We therefore designed a study to compare similar HiBALs and LoBALs at high resolution. We utilise the eMERLIN radio interferometer to obtain sub-arcsecond images of ten sources and study their radio properties on scales (Chapter 6). We find only one of the sources is resolved, indicating very compact emission for these BALQSOs. LoBALs appear to retain a greater fraction of their flux at high resolution although this is tempered by having to extrapolate fluxes from low frequencies for many of the sources. For sources with similar wind properties, the red sources appear to be brighter and retain a greater fraction of their flux.

I conclude by outlining the key results of the work within the thesis and providing an overall interpretation. I look to the future where I can build on this work and anticipate key developments in the field.