The Nature of Jets in Powerful Radio Galaxies
Relativistic plasma ejected from SMBHs at the centres of galaxies is known to play a key role in the AGN feedback cycle, and consequently the formation and evolution of structure in the universe. However, the physics driving the observed jet structure in these cosmic outflows remains an open question. As part of a project that aims to resolve some of these key questions in extragalactic jet physics, this thesis presents high resolution and sensitivity studies on the nature of jets in two known powerful FR I radio galaxies (3C465 & 3C83.1B), using the first ever deep transverse resolved radio observations of these objects from e-MERLIN, and with complementary observations from the EVLA. Our radio images represent the highest resolution and sensitivity maps of 3C465 to date, and compared with previous observations, our highest resolution (e-MERLIN only) map shows that there exist multiple bright knotty structures at the base of the NW jet in 3C465. A detailed description of the overall radio structure is presented. We measure sidedness ratio of 14.85, and consequently derive lower and upper limit values of 0.5c and 61o respectively for the jet velocity, βj and angle to the line of sight, θ in 3C465. We also use matched-resolution images across two frequencies (~1.5 and ~8.5 GHz) to construct accurate, spatially resolved spectral index image of the radio source and consequently derive the characteristic spectra across the entire radio galaxy (jet, hotspots and plumes). Our results shows that the jet spectra are fairly constant (<αjet> = -0.7), and the observed spectral flattening within the first 4.42 kpc radius from the core coincides with the region hosting the bright knots, and is consistent with the site of X-ray particle acceleration observed at the base of the radio jet in previous studies. There is very little dispersion (<Δα> = -0.04) between the spectra of the two hotspot components, plausibly indicating that electron populations of the same ages are injected at these sites. Our spectral profiles suggest that the NW and SE plumes in 3C465 are approximately homologous structures, in spite of the striking asymmetry in their physical size and shape. We attribute this asymmetry in morphology to 2-D projection effects, while variations in mass injection and propagation in external pressure and density gradients in the two regions account for the comparatively steeper spectrum in the NW plume, <αNWp> = -1.43 compared with the SE plume, <αSEp> = -1.38. Overall, our results show that first-order Fermi process at mildly relativistic shocks is the most likely acceleration mechanism at play in the radio source 3C465; and consistent with previous works, we conclude that two plausible acceleration mechanisms; (a) when bulk flow speeds, βj ≥ 0.5, and (b) when flow speeds, βj are less than ≈ 0.5 exist in our radio sample. Whereas the first case can accelerate electrons to high Lorentz factors, the second scenario dominates at slower speeds and larger distances. As extension of this work, we aim to undertake a detailed study of the radio source 3C83.1B and statistically compare the synchrotron spectra in the two samples, in order to check for any discrepancy. We also note for further study, the so called deviations from power law spectra which are indicators of synchrotron ageing and plausible diagnostics of the acceleration mechanism in order to place greater constraints on the nature of jet-particle acceleration in these sources; and in addition, polarization studies to investigate the orientation and degree of ordering of magnetic field in driving the evolution of the radio jets in these powerful FR I sources.
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