Impact of extragalactic point sources on the low-frequency sky spectrum and cosmic dawn global 21-cm measurements

TL;DR

This paper models the contribution of extragalactic point sources to low-frequency sky spectra, highlighting their impact on cosmic dawn 21-cm measurements and proposing a method to mitigate associated biases.

Contribution

It develops a new model for extragalactic point source contributions using luminosity and correlation functions, aiding in accurate 21-cm signal extraction.

Findings

Point sources contribute a few kelvins to the sky spectrum at 50-200 MHz.

Their contribution is about 0.4% of the total foregrounds in the REACH experiment.

Modeling point sources as a power law with a running spectral index reduces systematic bias.

Abstract

Contribution of resolved and unresolved extragalactic point sources to the low-frequency sky spectrum is a potentially non-negligible part of the astrophysical foregrounds for cosmic dawn 21-cm experiments. The clustering of such point sources on the sky, combined with the frequency-dependence of the antenna beam, can also make this contribution chromatic. By combining low-frequency measurements of the luminosity function and the angular correlation function of extragalactic point sources, we develop a model for the contribution of these sources to the low-frequency sky spectrum. Using this model, we find that the contribution of sources with flux density $>10^{-6}\,$Jy to the sky-averaged spectrum is smooth and of the order of a few kelvins at 50--$200\,$MHz. We combine this model with measurements of the galactic foreground spectrum and weigh the resultant sky by the beam directivity of the conical log-spiral antenna planned as part of the Radio Experiment for the Analysis of Cosmic Hydrogen (REACH) project. We find that the contribution of point sources to the resultant spectrum is $\sim0.4\%$ of the total foregrounds, but still larger by at least an order of magnitude than the standard predictions for the cosmological 21-cm signal. As a result, not accounting for the point-source contribution leads to a systematic bias in 21-cm signal recovery. We show, however, that in the REACH case, this reconstruction bias can be removed by modelling the point-source contribution as a power law with a running spectral index. We make our code publicly available as a Python package labelled epspy.