Reconstructing the nature of the first cosmic sources from the anisotropic 21-cm signal

TL;DR

This paper presents a realistic simulation of the anisotropic 21-cm power spectrum across early cosmic history, showing its potential to reveal properties of the first cosmic sources through observable anisotropy patterns.

Contribution

It provides the first detailed simulation of the anisotropic 21-cm power spectrum over a wide range of early universe epochs, highlighting its diagnostic potential.

Findings

Anisotropy is large and measurable at most redshifts.

The shape of the power spectrum during cosmic heating reflects X-ray source distribution.

A redshift window exists where anisotropy is small, enabling model-independent source characterization.

Abstract

The redshifted 21-cm background is expected to be a powerful probe of the early Universe, carrying both cosmological and astrophysical information from a wide range of redshifts. In particular, the power spectrum of fluctuations in the 21-cm brightness temperature is anisotropic due to the line-of-sight velocity gradient, which in principle allows for a simple extraction of this information in the limit of linear fluctuations. However, recent numerical studies suggest that the 21-cm signal is actually rather complex, and its analysis likely depends on detailed model fitting. We present the first realistic simulation of the anisotropic 21-cm power spectrum over a wide period of early cosmic history. We show that on observable scales, the anisotropy is large and thus measurable at most redshifts, and its form tracks the evolution of 21-cm fluctuations as they are produced early on by Lyman-a radiation from stars, then switch to X-ray radiation from early heating sources, and finally to ionizing radiation from stars. In particular, we predict a redshift window during cosmic heating (at z ~ 15), when the anisotropy is small, during which the shape of the 21-cm power spectrum on large scales is determined directly by the average radial distribution of the flux from X-ray sources. This makes possible a model-independent reconstruction of the X-ray spectrum of the earliest sources of cosmic heating.