Strap on your Boötes: Unveiling AGN Feedback through High Resolution Morphological Studies with LOFAR-VLBI

Abstract

Through both observational and theoretical studies we know that Active Galactic Nuclei (AGN) play a key role in regulating galaxy evolution through a process known as AGN feedback. Although we know that this process is occurring in our Universe, it remains unclear how this feedback operates. One promising avenue of research for advancing our understanding of AGN feedback is the study of multi-phase outflows.

In this thesis, I focus on a single phase of these multi-phase outflows: warm ionised gas. By investigating the kinematics of the forbidden [O~] 5007 emission line, it is possible to trace ionised outflows to understand their impact on the host galaxy. Previous works have investigated the impact of these ionised outflows on galaxies, however it is uncertain what mechanism is causing these outflows. Interestingly, this phase of outflow has been found to show a strong connection with radio emission. The relationship between the [O~] emission line and, in particular, low-frequency radio emission forms the primary focus of this doctoral work.

Following a discussion of the relevant scientific background (Chapter 1) as well as an overview of the fundamentals of radio astronomy (Chapter 2), Chapter 3 presents an analysis based on the first Low Frequency Array Two Metre Sky Survey Deep (LoTSS Deep) release. Here, I investigate the low-frequency, deep radio emission of AGN and link this to the kinematics of the [O~] emission line via spectral decomposition of Sloan Digital Sky Survey (SDSS) spectra. In this published work, we demonstrate that AGN detected in LoTSS Deep are more likely to host an [O~] outflow compared to AGN without a detection, hence confirming the link between radio emission and [O~] outflows.

To explore the physical mechanism behind this enhancement, we first examine the radio excess of our AGN sample and find that the majority of sources are non-radio excess, and therefore classified as radio-quiet AGN. This allows us to rule out powerful radio jets as the origin of their emission. To further investigate the nature of the radio emission, we turn to morphological studies. However, 90 of our radio detected population are unresolved in LoTSS Deep, limiting our ability to determine the origin of their radio emission. High-resolution imaging is therefore essential to resolve these sources and advance our understanding of the radio emission in both radio-quiet AGN and the physical mechanism driving the link between [O~] outflows and radio emission.

In Chapter 4, we focus on obtaining sub-arcsecond, kiloparsec-scale radio morphologies of the AGN sample introduced in Chapter 3. To achieve this, we exploit the full capabilities of the International LOFAR Telescope (ILT) and present the first, wide-field, sub-arcsecond resolution image of the Botes Deep Field at 144~MHz. This represents only the third wide-field image produced using the international ILT stations with Very Long Baseline Interferometry (VLBI) techniques. The scarcity of such images highlights the significant challenges posed during the data reduction process. In this chapter, I discuss the primary obstacles encountered when reducing a ~6 field of view at low frequencies, and I outline the data reduction steps performed to generate the Botes image, along with two intermediate resolution images at 0.6and 1.2. Thanks to this image, I now have access to sub-arcsecond resolution imaging across all three LoTSS Deep DR1 field.

In Chapter 5, I build on the connection between low-frequency radio emission and [O~] outflows by locating the radio detected AGN population from Chapter 3 within the wide-field, sub-arcsecond resolution images of Botes (Chapter 4), Lockman Hole, and ELAIS-N1. I utilise the spectral decomposition of SDSS spectroscopy performed in Chapter 3 to now link [O~] outflows with kiloparsec-scale radio emission.

Due to the high-resolution nature of these images, radio emission that was unresolved in LoTSS Deep DR1 (6resolution) can now be resolved. This enables a deeper understanding of the origin of radio emission in radio-quiet AGN and allows us to investigate the physical mechanism driving the connection between [O~] and low-frequency radio emission. Using a combination of sub-arcsecond morphological studies and brightness temperature measurements, I extend this investigation to a novel regime.

I find that AGN detected on both small (0.3) and large (6) scales are more likely to host [O~] outflows than AGN detected only on large scales. Considering that kiloparsec-scale radio emission is likely associated with the AGN, whereas diffuse large-scale emission is likely due to star formation, we conclude that the enhanced [O~] emission observed in these sources is predominantly AGN-driven.

I conclude this thesis in Chapter 6 by summarising the key outcomes of my doctoral work and by looking ahead to the promising future of statistically significant spectroscopic samples of radio detected sources, as well as forthcoming revolutionary radio surveys and instruments. These developments are expected to enable substantial scientific progress in the near future.