Early science with the murchison widefield array : from the moon to the nearest radio galaxies

Abstract

This thesis reports on new scientific results gleaned from test observations of well-known astronomical sources with the Murchison Widefield Array (MWA) telescope. Presented are three self-contained papers that demonstrate the capabilities of this recently commissioned radio interferometer array and provide new insights into the astrophysics of the Moon, Centaurus A and Fornax A at low frequencies. One of the main scientific goals of the MWA is the detection of the redshifted 21-cm line of neutral hydrogen from the epoch of reionization (EoR). To investigate the possibility of using the Moon as a calibration source for EoR detection and to test novel observing modes for the MWA, the Moon was observed over a frequency range of 80-170 MHz with the MWA 32-tile prototype. The observations show that the Moon's radio emission is as expected for a cool thermally emitting body, but that the spectrum is corrupted by reflected transmissions from Earth. In particular, the observed flux density of the Moon abruptly increases in the internationally recognized frequency modulated (FM) radio band. The implications for EoR detection using the Moon as a spectrally smooth reference source are discussed and the flux density of the Moon in the FM radio band is used to estimate the Earth's radio leakage and its potential for detection by an extraterrestrial race. Observations of our closest neighbouring radio galaxy, Centaurus A, were made at 118 MHz with the MWA 32-tile prototype. The resulting images demonstrated the widefield imaging capabilities of the MWA and provided the first evidence in total intensity for a southern counterpart to the well-known northern middle lobe of Centaurus A. A detailed spectral analysis of the radio lobes between 118 MHz and 1.4 GHz reveals significant spatial variation of the spectral index across the lobes, supporting a scenario of multiple outbursts of central activity being responsible for the large-scale morphology. Another nearby radio galaxy, Fornax A, was observed with the complete 128-tile MWA at 154 MHz. The MWA data were used in combination with radio, microwave, X-ray and gamma-ray data to model the spectral energy distribution of Fornax A and investigate, for the first time, the origin of the gamma-ray emission. It was found that the multi-wavelength data are poorly fit by a purely leptonic, inverse-Compton scattering model and that hadronic processes may account for the gamma-ray emission from Fornax A. Show more