A significant challenge to detecting the weak signal from the Epoch of Reionisation (EoR) is removing all of the foreground emission from bright extragalactic sources. February 2020’s Monthly Media is from Dr Christene Lynch, and highlights the many thousands of extragalactic sources which produce radio emission at the frequencies being searched for the EoR signal. Christene created the below movie by mosaicing images from the Murchison Widefield Array (MWA) in its new, extended configuration.

To remove the bright foreground emission produced by these extragalactic sources, the MWA EoR experiment requires accurate models of their radio emission. Christene creates these models from the still images in the movie. In order to ensure that these models are highly accurate, several tests are performed on the images to ensure that the positions and radio brightness of each source in the image is correct. An example of one of these tests is shown in the below image.

To test the source positions in the MWA images, Christene first identifies the brightest, most isolated point sources and matches them to other radio catalogues. These catalogues report positions and radio brightnesses at similar frequencies observed by the MWA. Differences between the catalogue position and that of the MWA are measured for each of the bright sources. These difference are shown in the above image, where Dec and RA are directions on the sky. The majority of the sources in the images are within a few 10’s of arcseconds from those reported in other catalogues — meaning that the source positions in the MWA images are quite accurate, and the images can be used for modelling!