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Title: A multi-wavelength study of radio-loud active galactic nuclei
Author: Mannering , Elizabeth J. A.
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2013
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In part I. I construct a sample of radio-loud active galactic nuclei (AGN) host galaxies from the optical Sloan Digital Sky Survey (SDSS) and Faint Images of the Radio Sky at Twenty cm (FIRST). These have 1.4GHz luminosities in the range 1023 - 1025 WHZ-l, span redshifts 0.02 < z < 0.18, are brighter than rpetro < 17.77 mag and are constrained to 'early-type' morphology in colour space (u• - r• > 2.22 mag). Optical emission-line ratios (at > 311) are used to remove type 1 AGN and star-forming galaxies from the radio sample using BPT diagnostics. For comparison, a sample of radio-quiet galaxies with the same r• -band magnitude-redshift distribution as the radio sample is selected. I also create comparison radio and control samples derived by adding the NRAO VLA Sky Survey (NVSS), to quantify the effect of completeness on the results. I investigate the effective radii of the surface brightness profiles in the SDSS r and u bands in order to quantify any excess of blue colour in the inner region of radio galaxies. A ratio is defined, R = r e( r) Ir e( u), and maximum likelihood analysis is used to compare the average value of R and its intrinsic dispersion between samples. R is larger for the radio-loud AGN sample as compared to its control counterpart, and I conclude that the two samples are not drawn from the same population at > 99% significance. I discuss host galaxy features that cause the presence of a radio-loud AGN to increase the scale size of a galaxy in red relative to blue light, including excess central blue emission driven by star formation, point-like blue emission from the AGN itself, and/or diffuse red emission. I favour an explanation that arises from the stellar rather than the AGN light. Given that iii star formation proceeds over a longer time than radio activity, the difference suggests that a subset of galaxies has a structured predisposition to become radio loud. In Part 2, I report results from Chandra and XMM-Newton observations of seven nearby (z < 0.07) radio galaxies. Each source shows evidence of an X-ray emitting gas belt, 011hogonal to and lying between the radio lobes, coincident with a deficit in radio emission. Detection of weak emission from a second, more ex tended group-type environment is also reported, as well as inverse-Compton X-ray emissions from the radio lobes in six of the sources. The X-ray emission from the nuclei correlates well with the core 5 GHz flux densities, supporting an emission origin at the base of the jet. Three high-ionization narrow-line radio galaxies in the sample show evidence for an additional absorbed component, consistent with the model in which they are fuelled by a standard thin accretion disk, heavily absorbed in X-rays when edge-on to the observer. The work extends previous detection of lobe inverse-Compton X-ray emission into the FRI regime, and the magnetic-field estimates shows that these systems are close to equipartition, or slightly electron dominated, as is found for FR II sources in other studies. The origin of the gas belts is diverse. The work concludes that two of the belts (those of NGC 7385 and NGC 6109) are galactic coronae, flattened by the expanding radio lobes. The belts of3C 192 and 4C73.08 are attributed to accumulated gas from merging groups of galaxies residing within the gas belt. For 3C 386, the belt is consistent with a hot gas halo surrounding an isolated field elliptical In the case of 3C 35 and 3C 98, the gas belt is interpreted as fossil -group gas driven outwards by the expanding radio lobes. The morphological structure of the radio lobes and gas belt point to co-evolution. Furthermore, the radio sources are powerful enough to eject galaxy-scale gas out to the edges of the gas belts, and the ages of the belts and radio structures are comparable in both sources. The destruction of 3C 35 and 3C 98's atmospheres may offer clues as to how fossil systems are regulated: radio galaxies may need to be within a particular range of intermediate powers to displace and regulate fossil-group gas. I discuss the implications of the gas belts in 3C 35 and 3C 98 in terms of AGN fuelling and feedback.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available