The Murchison Widefield Array
A view of part of the 128-element Murchison Widefield Array, which has been used to count the faint radio sources as astronomers prepare to explore the cosmic epoch of the first stars.
Counting up the number of objects of different kinds can help distinguish between models of how they form and evolve. Astronomers have traditionally used number counts of radio sources to learn about the nature of extragalactic source populations. The vast bulk of their radio continuum emission comes from a central active galactic nucleus and/or from ongoing star formation. The bright sources are relatively easy to identify, but they are rare -- more common are the moderate and low-power extragalactic radio sources. Extragalactic sources are located across a large range of cosmic distances, and because distant sources are fainter, they contribute particularly to the dim source counts. One of the major challenges for astronomers, therefore, is to get accurate statistics on sources that are intrinsically faint without counting in their company too many sources that are bright but just farther away. Number count statistics can do a good job of sorting out these possibilities.
The Murchison Widefield Array Square (MWA) is a low-frequency radio telescope, operating at meter-size wavelengths, and located at the Murchison Radio-astronomy Observatory (MRO) in Western Australia. It is a powerful survey instrument, an array of one hundred and twenty eight small telescopes, having both good sensitivity and a very wide field of view, and thus being an excellent tool for radio number counts. CfA astronomer Lincoln Greenhill was part of a large team that used the MWA to study faint source counts as part of a longer-term program to explore the universe as far back as the epoch of the first stars, only a few hundred million years after the big bang. In these first MWA observations, the team tested concepts and identified potential issues, including effects of the ionosphere and source confusion. Future studies will be undertaken with the very much larger Square Kilometer Array facility now under construction.
"The 154 MHz Radio Sky Observed by the Murchison Widefield Array: Noise, Confusion, and First Source Count Analyses," Franzen, T. M. O.; Jackson, C. A.; Offringa, A. R.; Ekers, R. D.; Wayth, R. B.; Bernardi, G.; Bowman, J. D.; Briggs, F.; Cappallo, R. J.; Deshpande, A. A.; Gaensler, B. M.; Greenhill, L. J.; Hazelton, B. J.; Johnston-Hollitt, M.; Kaplan, D. L.; Lonsdale, C. J.; McWhirter, S. R.; Mitchell, D. A.; Morales, M. F.; Morgan, E.; Morgan, J.; Oberoi, D.; Ord, S. M.; Prabu, T.; Seymour, N.; Shankar, N. Udaya; Srivani, K. S.; Subrahmanyan, R.; Tingay, S. J.; Trott, C. M.; Webster, R. L.; Williams, A.; Williams, C. L., MNRAS 459, 3314, 2016.