Browsing by Author "Taylor, A"

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    Broad-band radio polarimetry of disc galaxies and AGN with KAT-7
    (Oxford University Press, 2020-10-24) Taylor, A; Legodi, L; Stil, J
    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.
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    The evolution of the low-frequency radio AGN population to z 1.5 in the ELAIS N1 field
    (Advance Access publication, 2020-11-13) Ocran, E; Taylor, A; Vaccari, M
    We study the cosmic evolution of radio sources out to z 1.5 using a GMRT 610 MHz survey covering ∼1.86 deg2 of the ELAIS N1 field with a minimum/median rms noise 7.1/19.5 μJy beam−1 and an angular resolution of 6 arcsec. We classify sources as star forming galaxies (SFGs), radio-quiet (RQ) and radio-loud (RL) Active Galactic Nuclei (AGNs) using a combination of multiwavelength diagnostics and find evidence in support of the radio emission in SFGs and RQ AGN arising from star formation, rather than AGN-related processes. At high luminosities, however, both SFGs and RQ AGN display a radio excess when comparing radio and infrared star formation rates. The vast majority of our sample lie along the SFR − M ‘main sequence’ at all redshifts when using infrared star formation rates. We derive the 610 MHz radio luminosity function for the total AGN population, constraining its evolution via continuous models of pure density and pure luminosity evolution with ∝ ( 1 + z) (2.25±0.38)−(0.63±0.35)z and L610 MHz ∝ ( 1 + z) (3.45±0.53)−(0.55±0.29)z , respectively. For our RQ and RL AGN, we find a fairly mild evolution with redshift best fitted by pure luminosity evolution with L610 MHz ∝ ( 1 + z) (2.81±0.43)−(0.57±0.30)z for RQ AGN and L610 MHz ∝ ( 1 + z) (3.58±0.54)−(0.56±0.29)z for RL AGN. The 610 MHz radio AGN population thus comprises two differently evolving populations whose radio emission is mostly SF-driven or AGN-driven, respectively.
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    The global magneto-ionic medium survey: polarimetry of the southern Sky from 300 to 480 MHz
    (Ithaca: Cornell University Library, 2021) Wolleben, M; Landecker, T; Taylor, A
    The Galactic interstellar medium hosts a significant magnetic field, which can be probed through the synchrotron emission produced from its interaction with relativistic electrons. Linearly polarized synchrotron emission is generated throughout the Galaxy and, at longer wavelengths, modified along nearly every path by Faraday rotation in the intervening magneto-ionic medium. Full characterization of the polarized emission requires wideband observations with many frequency channels. We have surveyed polarized radio emission from the Northern sky over the range 1280–1750 MHz, with channel width 236.8 kHz, using the John A. Galt Telescope (diameter 25.6 m) at the Dominion Radio Astrophysical Observatory, as part of the Global Magneto-Ionic Medium Survey. The survey covered 72% of the sky, decl. −30° to +87° at all R.A. The intensity scale was absolutely calibrated, based on the flux density and spectral index of Cygnus A. Polarization angle was calibrated using the extended polarized emission of the Fan Region. Data are presented as brightness temperatures with angular resolution 40′. Sensitivity in Stokes Q and U is 45 mK rms in a 1.18 MHz band. We have applied rotation measure synthesis to the data to obtain a Faraday depth cube of resolution 150 rad m−2 and sensitivity 3 mK rms of polarized intensity. Features in Faraday depth up to a width of 110 rad m−2 are represented. The maximum detectable Faraday depth is ±2 × 104 rad m−2 . The survey data are available at the Canadian Astronomy Data Centre.