Exploring Radio-Loud AGN with Large Datasets

In this thesis, I use recent high sensitivity surveys that provide multi-wavelength information, to study and explore the key phenomenon of Radio-loud Active Galactic Nuclei (RLAGN). This thesis carries out in-depth investigations of three core aspects of these objects in order to understand the properties and evolution of AGNs. The studies herein make use of recent high sensitivity and resolution surveys, such as MIGHTEE, superMIGHTEE, LOFAR, and GMRT, to achieve better constrained estimates of spectral age distribution, uncover robust trends in the spectral index, redshift, and radio luminosity relationship, and critically test the unification model using new methods. In the thesis I have used the new sensitive and high-resolution MeerKAT MIGHTEE survey of the XMM-LSS field, along with data from the Low Frequency Array (LOFAR), the Giant Meterwave Radio Telescope (GMRT), and upgraded GMRT (uGMRT) surveys. These datasets include radio images and measurements at several frequencies such as 144, 390, 460, 610, 688, and 1400 MHz along with their narrow band, broad band, and deep field information. The first study focuses on the spectral age distribution of RLAGNs in the XMM-LSS field. This is important as jets of energetic particles, seen in FR type-I and FR type-II sources, ejected from the centre of radio-loud AGN affect the sources’ surroundings and the intracluster medium/ intergalactic medium. Placing constraints on the age of such sources is important in order to measure the jet powers and determine the effects on feedback. To evaluate the age of these sources I have used spectral aging models where high-resolution multi-wavelength data is required, which is provided by the MIGHTEE, LOFAR, and GMRT surveys. I present a sample of 28 radio galaxies with their best-fitting spectral age distribution analyzed using the Jaffe–Perola (JP) model on a pixel-by-pixel basis. The fits are generally good, and the objects in our sample show maximum ages within the range of 2.8 to 115 Myr with a median of 8.71 Myr. Some key conclusions of the study include that the use of high-resolution maps over a range of frequencies are required to observe detailed age distributions for small sources, and that high-sensitivity maps will be needed in order to observe fainter extended emission. In the study I also explore other parameters such as size and age correlation and comparison with dynamical age models. I do not observe any correlation between the total physical size of the sources and their age, and speculate that both dynamical models and the approach to spectral age analysis may need some modification to account for observed discrepancies in the results of the two models. The outcomes of the study can be used to place constraints on jet powers and comment on the extent of their influence on the environment. In the second study, I explore the trend previously observed between spectral index (α) and redshift for high-redshift radio galaxies (HzRG) and investigate the overall trend observed between spectral index, redshift, and radio luminosity along with the trends observed in previous studies. The study uses data from the new sensitive and high-resolution MeerKAT MIGHTEE survey of the XMM-LSS field, along with data from the Low Frequency Array (LOFAR), the Giant Meterwave Radio Telescope (GMRT), and upgraded GMRT (uGMRT) surveys, at frequencies 144, 390, 610, 688, and 1400 MHz, where the data is used for both pairwise and multi frequency analysis. The apparent trend observed between spectral index and redshift has been used for many years to select high-redshift objects by searching for radio sources with steep spectra. I use the surveys above to investigate this relationship in detail by selecting compact sources over a wide range of redshifts and luminosities. My results suggest that there is a correlation between α and z in the observed sample over some frequency range pairs, although there is a clear offset between the α-z relations in the observed sample and those derived previously from samples of more luminous objects. The relationships between α and luminosity are also weak in the observed sample but in general the most luminous sources are steeper-spectrum and this trend is extended by samples from previous studies. In detail, I have argued in the study about both a α-luminosity relation and an α-z relation which can be found in the data, but it is the former that drives the apparent α-z relation observed in earlier work, which only appears because of the strong redshift-luminosity relation in bright, flux density-limited samples. One of the conclusions of the study is that steep-spectrum selection must be applied with caution in searching for high-z sources in future deep surveys. A careful selection of HzRG incorporating effects of luminosity can further help to select unbiased complete population of sources, to be used for studying the properties of such galaxies and the effect they have in the early universe. In the final study, I test and investigate the orientation based unification model that argues that radio loud quasars and radio galaxies are part of the same population, observed at different angles. In this study I use the data provided by the LoTSS deep field survey in the Lockman Hole and ELAIS-N1 field. The key predictions of the model are that quasars are seen at smaller angles to the line of sight and so should be more projected, and hence apparently physically smaller, than corresponding radio galaxies, but this has not always been found to be the case in earlier studies. Using data from the LOFAR LoTSS deep fields, I test the unification model by measuring the critical angle and linear size ratio of quasars to radio galaxies for different redshift bins. I also argue that the interpretation of my (and all others’) observations requires a less simplistic model for the effects of projection, which takes into account the fact that real radio sources are of finite width and have an intrinsic axial ratio distribution. Using this model, I present simulations that predict the size ratio of observed lengths in the presence of a distribution of intrinsic physical sizes and axial ratios that are derived from observation, and compare the outputs with the observed results from the LoTSS data. For homogeneously selected sample in the study, the sample size is relatively small and the naive interpretation of the linear size ratio is not always consistent with the simplest expectations from the unified model. However, simulations show that the LoTSS sample, in common with every other sample so far used in the literature, is too small to carry out a reliable test. I conclude that to test the unified scheme, the sample size should be greater than ∼ 500 sources and homogeneously selected samples must be compared with the expectations from a realistic projection model. Using realistic models for testing orientation based unification scheme further helps modeling of AGN feedback which is known to affect the environment, and should also lead to a more refined understanding of processes involved in AGN activity.