Warm dark matter and the ionization history of the Universe

Abstract

In warm dark matter scenarios structure formation is suppressed on small scales with respect to the cold dark matter case, reducing the number of low-mass halos and the fraction of ionized gas at high redshifts and thus, delaying reionization. This has an impact on the ionization history of the Universe and measurements of the optical depth to reionization, of the evolution of the global fraction of ionized gas and of the thermal history of the intergalactic medium, can be used to set constraints on the mass of the dark matter particle. However, the suppression of the fraction of ionized medium in these scenarios can be partly compensated by varying other parameters, as the ionization efficiency or the minimum mass for which halos can host star-forming galaxies. Here we use different data sets regarding the ionization and thermal histories of the Universe and, taking into account the degeneracies from several astrophysical parameters, we obtain a lower bound on the mass of thermal warm dark matter candidates of mX > 1.3 keV, or ms > 5.5 keV for the case of sterile neutrinos non-resonantly produced in the early Universe, both at 90% confidence level. I. INTRODUCTIONThe appearance of the first generation of galaxies, when the Universe was a few hundred million years old, lead to the end of the so-called dark ages of the Universe. The ultraviolet (UV) photons emitted in these galaxies, gradually ionized the neutral hydrogen which had rendered the Universe transparent following the epoch of recombination, in a process known as reionization [1]. However, so far, the exact moment when cosmic reionization took place is not precisely known [2].The reionization transition in the late Universe increases the number density of free electrons, n e , which can scatter the Cosmic Microwave Background (CMB), with a probability related to the optical depth at reionization, τ , i.e., the line-of-sight integral of n e weighted with the Thomson cross section, and dominated by single-ionized hydrogen and helium states. The effect of free electrons on the CMB temperature anisotropies leads to a suppression of the acoustic peaks by a factor e −2τ at scales within the horizon at the reionization period, a signature which is very degenerate with the amplitude of the primordial power spectrum, A s . Nevertheless, the reionization process creates linear polarization on the CMB spectrum due to the scattering between free electrons and the large-scale CMB quadrupole. This signature, usually dubbed as the "reionization bump", with the induced polarized power scaling as τ 2 (see, e.g., Fig. 2 of Ref.[3]), and peaks at scales larger than the horizon size at the reionization period, resulting in a determination of τ almost free of degeneracies (see Ref. [3] or corresponding chapter in Ref. [2] for an exhaustive description of the epoch of reionization (EoR) and its impact on the CMB). Measurements by the Wilkinson Microwave Anisotropy Probe (WMAP) of the optical depth to reionization, τ = 0.089 ± 0.014, indicated an early-reionization scenario ...

New Model of Density Distribution for Fermionic Dark Matter Halos

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QCD Axion Dark Matter with a Small Decay Constant

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