Broad-band radio polarimetry of disc galaxies and AGN with KAT-7

Abstract

We report broad-band (1.2–1.9 GHz) radio continuum observations at arcminute resolutions of two nearby disc galaxies, NGC 1808 and NGC 1097, and four active galactic nuclei (AGN)-powered radio sources: PKS B1934−638, PKS B0407−658, J0240−231, and J0538−440. We use rotation measure synthesis to analyse their Faraday complexity. Observations were made with the seven-dish Karoo Array Telescope (KAT-7) radio telescope array, in South Africa. The AGN-powered sources fall into two ‘Faraday’ categories – simple and complex. The most polarized sources, J0538−440 and J0240−231, are found to have complex Faraday spectra that can be time variable (J0538−440 case) and also indicative of complex Faraday emitting and rotating components along the line of sight. PKS B0407−658 shows a simple Faraday spectrum, while PKS B1934−638 is undetected in polarization. The disc galaxies are classified as complex, albeit at low signal-to-noise ratio. This may indicate depolarization due to turbulence of the magnetized plasma in the bar and circumnuclear regions and/or frequency-dependent depolarization at L band.