Wide-Field Radio Surveys: Exploring the faint (microJy) Extragalactic Radio Source Population with the e-MERLIN, VLA, EVN and VLBA.

  • Ann Njeri Ng'Endo

Student thesis: Phd


One of the enduring and most fundamental questions of modern astrophysics is understanding how galaxies form and evolve through cosmic time. Key to this question is understanding the interplay and feedback between star-formation (SF) and accretion onto supermassive black holes (SMBHs). High sensitivity, high angular resolution radio observations are a crucial part of these investigations, uniquely providing spatially resolved tracers of SF and accretion (Active Galactic Nuclei, AGN) in high redshift galaxies. Deep wide-field radio surveys of extragalactic legacy fields trace a large range of spatial scales and brightness temperature sensitivities, with sensitivities to the different radio-emitting components in galaxies at cosmologically-significant distances. At ~GHz frequencies, arcsecond angular resolution scales offer a view of the diffuse radio emission largely associated with star formation regions (e.g., Condon et al., 2012; Vernstrom et al., 2015), with sub-arcsecond angular resolutions sensitive to and able to spatially resolve emissions from both AGN and star formation (e.g., Muxlow et al., 2020), while milliarcsecond angular resolution scales are mostly sensitive to compact emissions mainly associated with AGN (e.g., Herrera Ruiz et al., 2017; Radcliffe et al., 2018, 2021b, Njeri et al., 2022). At sub-milliJansky levels, these high redshift radio source populations are composed of a high fraction of star forming regions and radio quiet AGN with radio emissions at sub-kpc spatial scales (Orienti et al., 2015). For a better understanding of AGN and their host galaxies, a census of AGN at sub-mJy flux regimes is necessary. Therefore, the goal of this PhD, is to disentangle AGN contribution from SF processes over a range of redshifts, using unprecedented large field-of-view, ultra-deep and high-resolution radio imaging (0''.007-1''.6)provided by the European VLBI Network (EVN), the Very Long Baseline Array (VLBA), the enhanced Multi-element Radio Linked Interferometer (e-MERLIN) and the Karl G. Jansky Very Large Array (VLA). Combining these spatial scales, we provide a unique census of AGN identification through; i) radio morphology, ii) source sizes and flux density distribution, iii) brightness temperature, iv) variability, and v) multi-wavelength analysis such as radio-excess via the radio-far infrared correlation. The majority of our radio-detected AGN host galaxies are radio-quiet, with radio emission mostly being powered by a mix of star formation and AGN. By combining different spatial scales (parsecs to sub-kpc), we present a high spatial dynamic view of individual AGN galaxies at z > 1 and offer a direct view of the transition between compact radio emission and SF emission, important for star formation rates (SFR) measurements. Furthermore, we detect a possible rare dual AGN (potentially a binary SMBH) system. Ultimately, as progress is made in radio instrumentation, this work will provide a unique set of scientific and technical benchmarks for future high sensitivity high spatial resolution surveys of the static and dynamic radio sky at high redshifts.
Date of Award1 Aug 2023
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorMichael Garrett (Supervisor) & Rob Beswick (Supervisor)


  • Very Long Baseline Interferometry
  • Extragalactic
  • techniques: wide-field VLBI
  • galaxies: supermassive black holes
  • Active Galactic Nuclei
  • galaxies: active
  • techniques: interferometric
  • galaxies: high-redshift
  • techniques: high angular resolution

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