A new determination of the primordial helium abundance using the analyses of H ii region spectra from SDSS

ABSTRACT The precision measurement of the primordial helium abundance Yp is a powerful probe of the early Universe. The most common way to determine Yp is the analyses of observations of metal-poor H ii regions found in blue compact dwarf galaxies. We present the spectroscopic sample of 100 H ii regions collected from the Sloan Digital Sky Survey. The final analysed sample consists of our sample and HeBCD data base from Izotov et al. (2007). We use a self-consistent procedure to determine physical conditions, current helium abundances, and metallicities of the H ii regions. From a regression to zero metallicity, we have obtained Yp = 0.2462 ± 0.0022, which is one of the most stringent constraints obtained with such methods up to date and is in a good agreement with the Planck result $Y_{\rm p}^{\it {\mathrm{ Planck}}} = 0.2471 \pm 0.0003$. Using the determined value of Yp and the primordial deuterium abundance taken from Particle Data Group (Zyla et al. 2020) we put a constraint on the effective number of neutrino species Neff = 2.95 ± 0.16, which is consistent with the Planck one Neff = 2.99 ± 0.17. Further increase of statistics potentially allows us to achieve Planck accuracy, which in turn will become a powerful tool for studying the self-consistency of the standard cosmological model and/or physics beyond.

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

ABSTRACT Observational measures of the primordial helium mass fraction, YP, are of interest for cosmology and fundamental particle physics. Current measures obtained from ${\text{H}\, \small {II}}$ regions agree with the Standard Model prediction to approximately 1 per cent 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 REionization-Limited HI Clouds (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 a comparison of 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.