Low-frequency images of our nearest neighbouring radio galaxy
31 January 2014
When the Murchison Widefield Array (MWA) in Western Australia looks up at the sky, it sees thousands of distant galaxies that emit radio waves with tremendous intensity. Most of these objects appear as unresolved points in the radio maps produced; however, some are large and extended, allowing scientists, such as CAASTRO PhD student Ben McKinley (ANU) and colleagues, to examine their complex structure in intricate detail.
The closest radio galaxy to Earth is Centaurus A, at a distance of about 12 million lightyears. Its proximity and intrinsic size mean that its giant radio lobes extend an angular distance across the sky equal to that of 14 full Moons placed side-by-side. Counter-intuitively, this has made studies of Centaurus A difficult with traditional radio telescopes, as it requires many pointings to image the entire source.
The wide field of view of the MWA allows observers to fit Centaurus A into a single ‘snapshot’ though, and this is precisely what the MWA team did in a recent publication in order to study the properties of this radio galaxy at low frequencies. The widefield images at 118 MHz produced by the researchers were compared to previously published images at higher radio frequencies with some interesting results.
They found tentative evidence for the existence of a new structure in the southern giant lobe of Centaurus A that had previously only been detected in linear polarisation measurements. This was made possible by the low operating frequency of the MWA and the high image quality close to the bright core of Centaurus A. The team also examined the shape of the radio spectrum of the lobes and its variation across their spatial extent. The results support the widely held view that Centaurus A is a ‘restarting’ radio galaxy, with a central active galactic nucleus that has stopped and restarted its activity several times in its 80-million-year history.
by B. McKinley