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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 534; no. 2; pp. 1317 - 1328
• Main Authors: Mittal, Shikhar, Kulkarni, Girish, Anstey, Dominic, de Lera Acedo, Eloy
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 01-10-2024
• Subjects: Angular correlation; Beams (radiation); Bias; Clustering; Directivity; Flux density; Frequency measurement; Log spiral antennas; Luminosity; Point sources; Signal reconstruction; Spiral antennas; methods: data analysis; early Universe; dark ages, reionization, first stars; cosmology: theory
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stae2111

ABSTRACT
The 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 ${\gt} 10^{-6}$ 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 ${\sim}\, 0.4 {{\, \rm per\, cent}}$ 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.