Quantifying Foreground Contamination in the Dark Ages 21 cm Power Spectrum Using an Electromagnetically Simulated Dipole Antenna Atop a Dielectric Surface

Abstract

The highly-redshifted 21 cm signal from the cosmic Dark Ages presents an exciting frontier for cosmology, with the potential to observe a large number of Fourier modes of the cosmic density field in the absence of complicating astrophysical phenomena. Because the Earth's ionosphere and human-generated interference affect these low radio frequencies, the lunar far side - the most radio-quiet region in the inner solar system - is considered the ideal site to conduct such an experiment. The bright synchrotron foregrounds at these frequencies are expected to be spectrally smooth and thus occupy a subset of spectral Fourier modes, leaving an observable "window" to the Dark Ages signal. However, in practice, spectral structure arises in foregrounds due to instrumental artifacts, leading to a spillover of foreground power into the observable window. In this paper, we quantify this spillover for a set of simulated visibility measurements given a number of antenna beams, sky models, and baseline configurations. Notably, we examine the effects on foreground spillover due to variations in the lunar regoliths using two numerically simulated beam models with 1 and 4 layers of substrate materials underneath. We find that the regolith material properties can induce unwanted spectral structure in the beams, which potentially prevents foreground power suppression at the levels required. The detailed spectral behavior depends on the lunar regolith model used, making it paramount to produce both accurate models of foregrounds and of the lunar regolith for Dark Ages cosmology.