The imprint of warm dark matter on the cosmological 21-cm signal

Abstract

We investigate the effects of warm dark matter (WDM) on the cosmic 21-cm signal. If dark matter exists as WDM instead of cold dark matter (CDM), its non-negligible velocities can inhibit the formation of low-mass haloes that normally form first in CDM models, therefore delaying star formation. The absence of early sources delays the build-up of UV and X-ray backgrounds that affect the 21-cm radiation signal produced by neutral hydrogen. With use of the 21CMFAST code, we demonstrate that the pre-reionization 21-cm signal can be changed significantly in WDM models with a free-streaming length equivalent to that of a thermal relic with mass mX of up to ̃10-20 keV. In such a WDM cosmology, the 21-cm signal traces the growth of more massive haloes, resulting in a delay of the 21-cm absorption signature and followed by accelerated X-ray heating.CDMmodels where astrophysical sources have a suppressed photon-production efficiency can delay the 21-cm signal as well, although its subsequent evolution is not as rapid as compared to WDM. This motivates using the gradient of the global 21-cm signal to differentiate between some CDM and WDM models. Finally, we show that the degeneracy between the astrophysics and mX can be broken with the 21-cm power spectrum, as WDM models should have a bias-induced excess of power on large scales. This boost in power should be detectable with current interferometers for models with mX ≲ 3 keV, while next-generation instruments will easily be able to measure this difference for all relevant WDM models ©2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.