New catalogue details over 20,000 radio sources in southern sky
13 February 2017
Capturing extragalactic radio sources – predominantly radio galaxies – in large all-sky surveys provides us with crucial insights into how galaxies and their central super-massive black holes interact and evolve together over cosmic time scales. These radio source catalogues are invaluable tools, and combining catalogues at different frequencies gives us spectral information for many thousands of sources. This allows us to probe the physics producing the emission in the different galaxy populations.
The S-band Polarisation All-Sky Survey (S-PASS) is a total intensity and polarisation survey conducted with the CSIRO Parkes radio telescope at 2.3 GHz. The science goals for S-PASS include, but are not limited to, investigating Galactic and extragalactic magnetism, characterising the diffuse polarised Galactic synchrotron emission and the characterisation of polarised foregrounds for Cosmic Microwave Background science. One of the data products from this survey, which was completed in January 2010, is a set of 107 total intensity (Stokes I) maps covering the entire southern sky with ~11 arcminute resolution. As part of his Honours project, now-CAASTRO PhD student Bradley Meyers (Curtin University) and colleagues created an all-southern-sky radio source catalogue from this data.
The process of creating such a catalogue has a number of steps, each of which present their own challenges. Probably the most difficult step to get right is the source finding – how do we make a computer recognise a real radio source from, for example, a random noise bump? The researchers used the AEGEAN software package to find sources in all 107 of the tile images that map the sky. AEGEAN measures the background (the median) and the noise (the standard deviation) for each map to identify sources and characterise them by fitting an elliptical Gaussian model (which works very well for point sources). After filtering the candidate sources based on their signal-to-noise ratio and removing any multiple detections, the team created a catalogue of 23,389 sources from the S-PASS images, covering ~16,600 square degrees of sky (excluding the Galactic plane for latitudes <10 degrees).
The S-PASS total intensity source catalogue can provide new information to several areas of exciting science. For instance, the 2.3 GHz central frequency puts it in a good position to study Gigahertz Peaked Spectrum sources – peculiar objects thought to be the precursors to massive radio galaxies with a peaked spectrum around 1 GHz. Matching with other, similar resolution and frequency catalogues can reveal source variability over time scales of decades.