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

Sykes, Calvin and Fumagalli, Michele and Cooke, Ryan and Theuns, Tom (2020) 'Determining the primordial helium abundance and UV background using fluorescent emission in star-free dark matter haloes.', Monthly notices of the Royal Astronomical Society., 492 (2). pp. 2151-2160.


Observational measures of the primordial helium mass fraction, YP, are of interest for cosmology and fundamental particle physics. Current measures obtained from HII HII 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 YP 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 YP. 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 YP. 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.

Item Type:Article
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Publisher statement:This article has been accepted for publication in the Monthly notices of the Royal Astronomical Society ©: 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:13 December 2019
Date deposited:14 January 2020
Date of first online publication:20 December 2019
Date first made open access:14 January 2020

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