Interplanetary scintillation studies with the Murchison Widefield Array III: Comparison of source counts and densities for radio sources and their sub-arcsecond components at 162MHz

Abstract

© 2018 The Author(s). We useMurchisonWidefieldArray(MWA) observations of interplanetary scintillation (IPS) to determine the source counts of point (< 0.3 arcsec extent) sources and of all sources with some subarcsecond structure, at 162 MHz. We have developed the methodology to derive these counts directly from the IPS observables, while taking into account changes in sensitivity across the survey area. The counts of sources with compact structure follow the behaviour of the dominant source population above ~3 Jy but below this they show Euclidean behaviour. We compare our counts to those predicted by simulations and find a good agreement for our counts of sources with compact structure, but significant disagreement for point source counts. Using low radio frequency spectral energy distributions (SEDs) from the Galactic and Extragalactic MWA survey, we classify point sources as compact steep spectrum (CSS), flat spectrum, or peaked. If we consider the CSS sources to be the more evolved counterparts of the peaked sources, the two categories combined comprise approximately 80 per cent of the point source population. We calculate densities of potential calibrators brighter than 0.4 Jy at low frequencies and find 0.2 sources per square degrees for point sources, rising to 0.7 sources per square degree if sources with more complex arcsecond structure are included. We extrapolate to estimate 4.6 sources per square degrees at 0.04 Jy. We find that a peaked spectrum is an excellent predictor for compactness at low frequencies, increasing the number of good calibrators by a factor of three compared to the usual flat spectrum criterion.