Determining the primordial helium abundance and UV background using fluorescent emission in star-free dark matter haloes

TL;DR

This paper proposes a novel method to measure the primordial helium abundance and UV background spectral slope using hydrogen and helium emission lines from pristine, star-free dark matter haloes called RELHICs, offering an independent test of cosmological models.

Contribution

The study introduces the use of RELHICs as a new observational probe for primordial helium and UV background, supported by radiative transfer simulations demonstrating its feasibility.

Findings

RELHICs can be used to infer $Y_{ m P}$ from emission line comparisons.

Simulations show the potential to measure UV background spectral slope.

RELHICs provide an independent test of primordial helium abundance.

Abstract

Observational measures of the primordial helium mass fraction, $Y_{\rm P}$, are of interest for cosmology and fundamental particle physics. Current measures obtained from H II regions agree with the Standard Model prediction to approximately 1% precision, although these determinations may be affected by systematic uncertainties. This possibility can only be tested by independently measuring the helium abundance in new ways. Here, we propose a novel method to obtain a measurement of $Y_{\rm P}$ using hydrogen and helium recombination line emission from RELHICs: pristine, gas-rich but star-free low-mass dark matter haloes whose existence is predicted by hydrodynamical simulations. Although expected to be uncommon and intrinsically faint in emission, the primordial composition and simple physical properties of these objects make them an ideal laboratory to determine $Y_{\rm P}$. We present radiative transfer simulations to demonstrate the effectiveness of this approach, finding that comparing the emission in H and He lines, either via their volumetric emissivities, or integrated properties such as the surface brightness and total flux, may be used to infer $Y_{\rm P}$. Furthermore, we show that RELHICs can be used to provide an entirely novel constraint on the spectral slope of the ultraviolet background, and discuss the possibility of measuring this slope and the primordial helium abundance simultaneously.