Shot noise and scatter in the star formation efficiency as a source of 21-cm fluctuations

TL;DR

This paper investigates how Poisson fluctuations and scatter in star formation efficiency influence the 21-cm signal during cosmic dawn and reionization, highlighting their significant impact on the power spectrum and implications for future observations.

Contribution

It quantifies the effects of star formation efficiency scatter and Poisson fluctuations on 21-cm fluctuations, an aspect often neglected in previous models.

Findings

Both effects significantly alter the 21-cm power spectrum.

Scatter introduces a feature related to galaxy brightness distribution.

Reduced correlation between density and radiation fields observed.

Abstract

The 21-cm signal from cosmic dawn and the epoch of reionization (EoR) probes the characteristics of the high redshift galaxy population. Many of the astrophysical properties of galaxies at high redshifts are currently unconstrained due to the lack of observations. This creates a vast space of possible astrophysical scenarios where the 21-cm signal needs to be modeled in order to plan for, and eventually fit, future observations. This is done with fast numerical methods which make simplifying approximations for the underlying physical processes. In this work we quantify the effect of Poisson fluctuations and scatter in the star formation efficiency; while Poisson fluctuations are included in some works and not in others, scatter in the star formation efficiency is usually neglected, and all galaxies of a given mass are assumed to have the same properties. We show that both features can have a significant effect on the 21-cm power spectrum, most importantly in scenarios where the signal is dominated by massive galaxies. Scatter in the star formation efficiency does not simply enhance the effect of Poisson fluctuations; for example we show that the power spectrum shape at cosmic dawn has a feature corresponding to the width of the galaxy brightness distribution. We also discuss some of the consequences for 21-cm imaging, and the signature of reduced correlation between the density and radiation fields.