Decomposing quasar radio emission in galaxy evolution across cosmic time

Abstract

Radio active galactic nuclei (AGN) play an important role in regulating the evolution of galaxies through feedback processes related to radio jet and outflow activities. Understanding the nature of quasar radio emission can help us clarify the details of AGN evolution stages and feedback processes, in the hope of answering the question of how AGN and host galaxies co-evolve through cosmic times. Although previous studies show that both radio jets from the AGN and the star formation (SF) activity in quasar host galaxies contribute to the quasar radio emission, their relative contributions across the population remain unclear, making it hard to identify the physical processes that govern the evolution of radio quasars.

In this thesis, I will present an improved Bayesian parametric model that allows us to statistically separate the SF and AGN components in the observed quasar radio flux density distributions, and to accurately measure the host galaxy SF and AGN jet contributions to quasar radio emission and their evolution with various physical processes, therefore putting new observational constraints on the current theories of jet powering, AGN evolution, and the role of AGN activities in galaxy evolution.

In Chapter 2, I will describe the theoretical foundation of the two-component parametric model, where I used a log-Gaussian distribution to model the host galaxy SF contribution to the total quasar radio flux density distribution and a single power-law distribution to model the AGN jet contribution. I have established a Bayesian method to fit the observational data to the model and have conducted various tests using mock data to validate the approach.

In the following scientific chapters, I will present the applications of my model to the novel data from the LOFAR Two-Metre Sky Survey Data Release 2 and the Sloan Digital Sky Survey, together with the results and understanding that arise from my analysis.

In Chapter 3, I demonstrate that the typical star formation rate of the quasar host galaxy increases with bolometric luminosity and with redshift to redshift ∼ 4. The prevalence of radio AGN emissions increases with quasar luminosity but has little dependence on redshift, indicating that the AGN jet power is mostly governed by local activities. My new methodology also allow for investigations on the roles of various physical parameters including quasar colour, black hole mass, and environment, on quasar radio emission. In Chapter 3, I will show that the radio excess in red quasars is due to an enhancement in AGN-related emission and that this radio enhancement occurs mostly in quasars with weak or intermediate radio power, linking red quasars to a special quasar evolutionary phase. In Chapter 4, I will present a coherent picture of the impact of black hole mass on quasar radio emission, where a full range of jet powers is seen at all black hole masses, and only quasars hosting the most massive black holes show an enhancement in radio emissions due to the higher incidence of powerful jets, which is potentially linked to the difference in accretion modes.

In Chapter 4, I will also show that the traditional radio-loud/quiet quasar classification fails to reflect the physical origin of radio emissions in each population, while my model allows for a refined definition of the radio AGN populations based on the jet or SF dominance in quasar radio emissions rather than naive flux- or luminosity-ratio cuts. My new definition unifies previously divergent observational results, highlighting the importance of classifying radio AGNs based on physical processes. In Chapter 5, under my new definition, I will discuss the impact of large-scale environment on quasar radio loudness, where I will show through analysis of auto-correlation functions that quasars dominated by jet activities reside in haloes ∼ 100 times more massive than those without strong jets, pinpointing the origin of powerful AGN jets to rich cluster environments. The results in Chapter 5 show no bimodality between the host dark matter halo masses of jet- an SF-dominated quasars and between quasars with different jet powers but rather a continuous evolution of clustering strengths, indicating that there is no minimum halo mass or BH mass required for the triggering of jets and that the halo mass is responsible for determining the power of radio jets.

Finally, in Chapter 6, I will discuss avenues for future work. The recently launched Euclid space telescope will broaden the scope of the work presented in this thesis thanks to its unique near-infrared (NIR) band coverage and unprecedented surface brightness sensitivity, leading to two potential follow-up projects to the thesis work: the first project will focus on the role of mergers in the triggering and power mechanism of radio jets, while the second project will aim to build a census of red and obscured quasars through a joint radio-NIR selection method. I will also discuss the applications of my model in the era of next-generation spectroscopic surveys using the multi-object WEAVE spectrograph on the William Herschel Telescope, and the revolutionary deep radio surveys to be carried out with LOFAR 2.0 instruments and the Square Kilometre Array (SKA). The methodology presented in this thesis has great potential in revealing the mechanisms behind quasar radio emissions across the extra parameter space to be explored by the SKA and its pathfinder missions and will become critical in defining and classifying the radio AGN populations in future surveys.