Wideband spectropolarimetry of extragalactic radio sources with KAT 7 and commissioning phase MeerKAT

Legodi, Letjatji S.

Wideband spectropolarimetry of extragalactic radio sources with KAT 7 and commissioning phase MeerKAT

Doctoral Thesis

2018

Abstract

The study of cosmic magnetism is vital in fully understanding the role that magnetic fields play in the evolution of cosmological objects such as galaxies. The exact role played by magnetism in galaxy evolution is still not fully understood. This work aims to use wide band radio polarimetry to probe the frequency dependent polarization properties in a small sample of unresolved known disc and AGN powered radio galaxies and also in a larger faint radio source sample from a deep blind survey. The second sample is composed of lesser known faint radio sources spanning ∼ 4 square degrees. We also aim to develop broad band radio polarimetry techniques that can be utilised in the future. We observed six radio galaxies at frequencies spanning 1200−1900 MHz – divided into three 256 MHz bands centred on 1350, 1600, and 1850 MHz. We also analysed deeper wide band and wide field observations from the commissioning phase of the South African SKA precursor, MeerKAT. The MeerKAT observations are of the DEEP field, a radio quiet region in the southern sky. These observations were made at frequencies 890 MHz to 1702 MHz. Following data calibrations, we made sensitive (down to ∼ 7 µJy/beam) high resolution (6”) total intensity image maps for DEEP field source finding and characterization, as well as lower resolution (14”) IQU image cubes. We analysed sources with total flux density greater than one milliJy. The technique of Faraday rotation measure synthesis (RM synthesis) was employed through a python based algorithm to probe the broad band structure of the polarisation spectra. RM synthesis has the power to resolve individual contributions to the overall observed spectropolarimetric features along a line of sight. We found agreement with literature with regard to total radio intensities and the expected polarisation levels of order 1%. Our polarisation detection rate in the wide field case was estimated to be 24 ± 9% per square degree. This translates to 30 ± 13 polarised sources per square degree, in agreement with projections from the literature. The results confirm two main categories of polarised radio sources. These are: (1) sources with simple polarisation profiles, theoretically resulting from a scenario where incident polarised emission is Faraday rotated by a foreground region of coherent magnetic field such as in the case of a radio bright galactic nucleus emitting synchrotron radiation that then traverses disk and halo regions filled with a uniform magnetic field; (2) sources with complex Faraday spectra, indicating depolarisation and/or repolarisation suggesting other complex behaviour originating from several combinations of Faraday rotating and synchrotron emitting regions along the line of sight. We found that the most complex polarisation profiles emanate from the most polarised radio sources. From the sample of known sources observed with KAT 7, we found that unresolved disk dominated galaxies tended to be less polarised than compact sources suggesting a morphological dependence of the polarisation spectrum where disk galaxies may tend to have more regions within them, such as pockets of turbulent magnetic fields in the material disk, that may cause depolarisation to a larger degree as compared to earlier type non-disk dominated galaxies. Steep spectrum sources of different types (radio lobes and most polarised compact sources) show the most complexity with multiple Faraday emission components, depolarisation, and repolarisation. The more sensitive MeerKAT data allowed us to probe the broad band polarimetric properties of faint never observed radio sources. Our study is an early step to future campaigns with MeerKAT to explore relations between polarimetry and source properties, probing the correlations of magnetism with galaxy evolution and the evolution of large-scale fields in galaxies, galaxy groups and clusters.

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PhD / Doctoral

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