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

TL;DR

This study uses Murchison Widefield Array data to analyze the counts and densities of compact and sub-arcsecond radio sources at 162 MHz, improving understanding of source populations and calibrator availability.

Contribution

It introduces a methodology to derive source counts from IPS observables, accounting for survey sensitivity variations, and compares these counts with simulations, revealing new insights into source classifications.

Findings

Counts of compact sources follow known population trends above 3 Jy.

Significant disagreement between observed and simulated point source counts.

Peaked spectrum sources are strong indicators of compactness, boosting calibrator candidate numbers.

Abstract

We use Murchison Widefield Array observations of interplanetary scintillation (IPS) to determine the source counts of point ($<$0.3 arcsecond 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 $\sim$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 SEDs from the GLEAM 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% 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.