The Nature of Compact Sources Selected by LOFAR

Radio-loud active galactic nuclei (RLAGN) produce jets on large scales heating their local environments and preventing the hot phase of baryonic matter from cooling hence slowing or halting galaxy formation and evolution; a model popularly known as AGN feedback. To understand this model, new views of low-luminosity compact RLAGN inhabiting massive galaxies have been obtained using the new LOw Frequency ARray (LOFAR) northern-sky radio survey LoTSS (LOFAR Two Metre Sky Survey) to study RLAGN in the local Universe at low frequencies (150 MHz). In this thesis, I investigate lowluminosity compact RLAGN selected from LoTSS first Data Release (DR1) in an attempt to reveal their radio morphologies and the physical scales at which they affect the inter-galactic medium (IGM) of the host galaxy on sub-kpc scales. We have conservatively selected 55 low-luminosity compact RLAGN based on LoTSS DR1 within redshift range 0:03 < z < 0:1. I show using high-frequency Jansky Very Large Array (VLA) observations that only 9 out of 55 objects show extended radio emission (1-3 kpc), 43 are compact at the limiting angular resolution of 0.35 arcsec (corresponding to projected maximum physical sizes of < 1 kpc), while 3 are undetected. The extended objects display a wide range of radio morphologies on smaller angular scales: doubles (3), two-sided jets (3), one-sided jets (2), and complex (1). I discuss their radio spectra which range from steep to flat and/or inverted radio spectra (-1:31 < α < 0:36) and span the range seen in various compact RLAGN such as compact symmetric objects (CSOs), compact steep spectrum (CSS) sources, and gigahertz peaked-spectrum (GPS) sources. Assuming synchrotron self-absorption (SSA) models for flat and/or inverted radio spectrum sources, I predict the physical sizes of 35 RLAGN to be between 2-53 pc. On their position on the power-linear size (P - D) diagram, we see that these objects populate the bottom left lower-end of the diagram; this position corresponds to the low-radio power CSOs. Finally, I compare their radio properties with those of the Fanaroff-Riley class 0 (‘FR0’) sources. Interestingly, some of the compact RLAGN presented in this thesis show extended radio emission in the second Data Release (DR2) images; these appear to be in fact the radio cores of low-luminosity yet extended Fanaroff-Riley (FRI/FRII) sources that could populate the right-lower end of the P - D diagram, limited by LOFAR’s surface brightness. Could we be underestimating the sizes of some of these compact objects? Continued VLBI observations of LOFAR sources could help us answer this question, which in turn will help us further understand AGN feedback. i