부산대학교 도서관

Extragalactic relativistic plasma jets ejected from SMBHs lurking at the centres of massive galaxies are 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. In this thesis, I present high resolution and high sensitivity studies of the jet bases in a sample of six extragalactic radio sources, using deep transverse resolved radio observations of these objects from the e-MERLIN and LOFAR telescopes, and with complementary observations from the VLA along with optical (HST) and X-ray (Chandra) data. These observations provide unique probes of the dynamics and energetics of the inner jets. Physical parameters that can be derived from spectral data, along with relations between the observed quantities for a beamed radio jet, such as intrinsic velocity and orientation angle (βjet, θ), are investigated. The mapped objects are representative of the range of FR I morphological types in a low-redshift (z ≲ 0.05) flux-limited survey, but we also include a nearby BL Lac object because such objects are usually related to FR I sources by unification schemes. This work forms part of ongoing efforts by the e-MERLIN Jets Legacy programme to "resolve key questions in extragalactic jet physics." This thesis begins with a review of existing literature relevant to the study and the scientific motivation for undertaking the present inquiry (Chapter 1). I then present the data and instruments in Chapter 2, and provide a detailed description of the observations and methods employed in the present work in Chapter 3. Next I present results (Chapter 4) from 144 MHz LOFAR HBA observations of the kpc-scale jets in our sample. These high dynamic range (DR ≳ 10⁵:1) LOFAR HBA images allow us to characterize the source morphological properties with high precision, and draw conclusions on the host galaxies and their environment. We also detect special features such as loops, and report for the first time the detection of a compact lobe (blob) at the bend in the main jet in the radio galaxy 3C 264. Our derived ~ 150-MHz radio luminosities are consistent with the six sources lying close to the FR I/FR II luminosity break, and show that the physical scales of some of the sources in our sample have been underestimated by a factor of a few in previous studies. I then present results on the energetics of the jet bases for three sources 3C 83.1B, 3C 264 and 3C 371 (Chapters 5, 6 and 7) from multifrequency radio observations. The inner jets in these sources are well resolved and there is good correspondence between features of the jet observed at all three frequencies. We estimate mildly relativistic speeds (ßjet = 0.2c) for the inner jets in 3C 83.1B and infer bulk deceleration of the flow at the jet base, most likely by entrainment. The value we find for synchrotron lifetime, Tsyn = 4.2×10⁷ years, suggest that relativistic electrons created in and around the central engine could travel the entire ~ 25 kpc length of the inner jets within their lifetime for bulk velocities of υ_bulk > 0.05c if there is no reacceleration or adiabatic losses. The transverse intensity distribution of the inner structure in 3C 264 suggests plausible velocity stratification within the inner relativistic jet as reported in previous studies. We measure an intrinsic speed of 0.42c (Tjet = 1.1) which constrains the jet viewing angle θ to assume relatively large values (52° ≲ θ ≲ 74°), and when compared with the higher initial VLBI jet velocities of ~ 0.99c (Γ = 7) for the inner 300–400 pc, a deceleration of the order of a factor 2 from the initial parsec-scale jet can be inferred. The radiative lifetime of electrons emitting at all three frequencies are very similar, at about 3 Myr. Our observations are consistent with 3C 371 being core-dominated, with the central core emitting ≳ 50% of the total flux across all three frequencies and we find evidence of double asymmetric radio knots located 1.3"–1.8" away from the compact core. We infer highly relativistic speeds, ßjet = 0.81c and an orientation angle, θ = 35° for the inner jets in 3C 371. The values we find for the radiative lifetime of electrons emitting at GHz frequencies is of the order of a few 10⁵ years, consistent with previous works and suggests that the jets in 3C 371 are relatively young compared with the other sources studied here. Future work suggested on this sample is described in Chapter 8.