Investigating the magnetic fields of high-velocity outflows from active galactic nuclei

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Richardson, Frederick
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University College Cork
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The focus of my M.Sc. degree was the magnetic fields of the jets of Active Galactic Nuclei (AGNs). The exact mechanism for the high levels of collimation in AGN jets is poorly understood. Theoretical models suggest that the collimation of jets is at least partially the result of a helical magnetic field. The primary aim of this thesis is to search for evidence of this structure. For such a magnetic field it is expected to find a Faraday rotation measure gradient transverse to the jet, an increase in fractional polarisation towards the jet edges, and possibly a "spine-sheath" polarisation angle structure. AGN jets that feature more than one of these criteria mentioned may host helical magnetic fields. Observations obtained in 2017 and 2019 of a sample of 16 AGNs (BL Lac objects) were analysed at four radio wavelengths: 6, 13, 17, and 21 cm. The data for these observations were obtained with the "Very Long Baseline Array" of ten radio telescopes spread across the United States. These were the first data of this kind for these AGNs. Observations at multiple radio wavelengths are needed to construct rotation measure maps and detect the presence of a helical or toroidal magnetic field. Eleven of the 16 BL Lac objects host convincing transverse rotation measure gradients with statistical significances greater than 3σ. Two sources ( J0738+1742 and J2202+4216) showed an increase in fractional polarisation towards both edges of the jet as well as a spine-sheath structure. In addition, J0748+2400, J0958+6533, J1800+7828, J1806+6949, J1824+5651, J2005+7752, and J2202+4216 had the EVPA and fractional polarisation structure discussed earlier which strengthens the argument for these jets possibly having helical fields. A secondary objective of my research was to compare my results with the jet structures of these AGNs observed at four wavelengths between 18 and 22 cm at two earlier dates in 2004 and 2010. The morphology, polarisation structure outside the core, and fractional polarisation remained approximately constant. Analogous statistically significant Faraday rotation measure gradients were found at these earlier dates in J0050-0929, J1806+6949, and J1824+5651, and evidence for transverse RM gradient reversals was found for J0818+4222, J0825+0309, J1800+7828, and J2202+4216. The reversal of the direction of a transverse RM gradient with distance from the core or time could result from a nested helical field structure, with the toroidal components of the helical fields being in opposite directions in the inner and outer regions of helical field.
Radio astronomy , Galaxies , Extragalactic , Jets , Quasars , Polarisation , Active galactic nuclei , Magnetic fields
Richardson, F. 2021. Investigating the magnetic fields of high-velocity outflows from active galactic nuclei. MRes Thesis, University College Cork.