Exploring Extended Radio Galaxies with LOFAR

Barkus, Bonny (2023). Exploring Extended Radio Galaxies with LOFAR. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.00016120

Abstract

Extended radio sources are an important minority population in modern, deep radio surveys because they enable detailed investigation of the physics governing radio-emitting regions such as active galaxies and their environments. The Low-frequency array (LOFAR) Two-metre Sky Survey (LoTSS), at 150 MHz, is the largest radio survey to date in terms of numbers of sources and data volume, and is sensitive to both compact and extended emission, making it ideal for the study of extended radio sources.

Cross-identification of radio sources with optical host galaxies is challenging for this extended population, due to their morphological complexity. In LoTSS Data Release 1 an automated statistical identification process for compact sources was supplemented by a citizen science identification process. I present a novel method for automating the host identification of extended sources by using ridgelines, which trace the assumed direction of fluid flow through the points of highest flux density. Applying my code, RL-XID, I demonstrate that ridgelines are versatile; they can be used both for optical host identification and morphological studies. RL-XID draws ridgelines for 85% of sources brighter than 10 mJy and larger than 15 arcsec, with an improved performance of 96% for the subset >30 mJy and >60 arcsec. I demonstrate that RL-XID successfully identifies the host for 98% of the sources with successfully drawn ridgelines, and performs at a comparable level to visual identification via citizen science.

Exploring galaxy populations helps us to understand their evolution and build better population models. I present new VLA observations that reveal the structure of a population of low-luminosity FRII radio galaxies discovered in LoTSS. Fanaroff and Riley (1974) identified a relationship between luminosity and radio structure, with FRIs having low-luminosity, centre bright jets and FRIIs having higher luminosity, edge-brightened jets. Using LoTSS, Mingo et al (2019) demonstrated an overlap in luminosity between FRI/FRII morphology, discovering a sub-sample of FRIIs with luminosities up to three magnitudes lower than the typical FR break. To continue to apply the traditional FR classifications in upcoming surveys we need to understand the structure and dynamics of the low-luminosity FRII population. The VLA observations I present in this thesis are a sample of LoTSS-selected low-luminosity FRIIs which allow me to make comparisons between the two FRII luminosity populations. I found systematic differences in the prevalence of compact features, with a higher fraction of cores in the low luminosity sample, and a higher fraction of hotspots in the high luminosity sources. I have also identified six new remnant candidates and three new restarting candidate sources.

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