Beamforming approaches toward detecting the 21-cm global signal from Cosmic Dawn with radio array telescopes

TL;DR

This paper explores beamforming techniques with large radio arrays as a promising alternative to traditional radiometers for detecting the faint 21-cm global signal from Cosmic Dawn, highlighting simulation results and systematic challenges.

Contribution

It demonstrates through simulations that beamforming with large arrays can effectively measure the 21-cm global signal, addressing systematic sidelobe effects and proposing a feasible array configuration.

Findings

Beamforming arrays can detect the 21-cm global signal despite sidelobe complexities.

Sidelobe structure introduces frequency-dependent systematics.

Arrays with ~10^5 antennas can achieve the necessary sidelobe suppression.

Abstract

The formation of the first stars and galaxies during 'Cosmic Dawn' is thought to have imparted a faint signal onto the 21-cm spin temperature from atomic Hydrogen gas in the early Universe. Observationally, an absorption feature should be measurable as a frequency-dependence in the sky-averaged (i.e. global) temperature at meter wavelengths. This signal should be separable from the smooth -- but orders of magnitude brighter -- foregrounds by jointly fitting a log-polynomial and absorption trough to radiometer spectra. A majority of approaches to measure the global 21-cm signal use radiometer systems on dipole-like antennas. Here, we argue that beamforming-based methods may allow radio arrays to measure the global 21-cm signal. We simulate an end-to-end drift-scan observation of the radio sky at 50--100 MHz using a zenith-phased array, and find that the complex sidelobe structure introduces a significant frequency-dependent systematic. However, the {\lambda}/D evolution of the beam width with frequency does not confound detection. We conclude that a beamformed array with a median sidelobe level around 50 dB below the main beam may offer an alternative method to measure the global 21-cm signal. This level is achievable by arrays with O(10^5) antennas.