# A Space-Based Observational Strategy for Characterizing the First Stars and Galaxies Using the Redshifted 21-cm Global Spectrum

## TL;DR

This paper proposes a lunar-orbit space-based observational strategy to detect the redshifted 21-cm global spectrum, aiming to probe the epoch of first stars and galaxies by overcoming terrestrial interference and foreground contamination.

## Contribution

It introduces a novel lunar-orbit observational approach with spectral modeling and data analysis techniques to measure the 21-cm signal from the early universe.

## Key findings

- Demonstrates the feasibility of detecting the 21-cm global signal from lunar orbit.
- Shows that foregrounds can be effectively separated using eigenmode analysis.
- Provides a framework for constraining early universe astrophysical parameters.

## Abstract

The redshifted 21-cm monopole is expected to be a powerful probe of the epoch of the first stars and galaxies ($10<z<35$). The global 21-cm signal is sensitive to the thermal and ionization state of hydrogen gas and thus provides a tracer of sources of energetic photons -- primarily hot stars and accreting black holes -- which ionize and heat the high redshift intergalactic medium (IGM). This paper presents a strategy for observations of the global spectrum with a realizable instrument placed in a low altitude lunar orbit, performing night-time 40-120 MHz spectral observations, while on the farside to avoid terrestrial radio frequency interference, ionospheric corruption, and solar radio emissions. The frequency structure, uniformity over large scales, and unpolarized state of the redshifted 21-cm spectrum are distinct from the spectrally featureless, spatially-varying, and polarized emission from the bright foregrounds. This allows a clean separation between the primordial signal and foregrounds. For signal extraction, we model the foreground, instrument, and 21-cm spectrum with eigenmodes calculated via Singular Value Decomposition analyses. Using a Markov Chain Monte Carlo algorithm to explore the parameter space defined by the coefficients associated with these modes, we illustrate how the spectrum can be measured and how astrophysical parameters (e.g., IGM properties, first star characteristics) can be constrained in the presence of foregrounds using the Dark Ages Radio Explorer (DARE).