# Warm dark matter and the ionization history of the Universe

## TL;DR

This paper investigates how warm dark matter influences the Universe's ionization history, using observational data to constrain the dark matter particle mass despite astrophysical uncertainties.

## Contribution

It provides new constraints on warm dark matter particle mass by analyzing ionization and thermal history data while accounting for parameter degeneracies.

## Key findings

- Lower bound on thermal warm dark matter mass: >1.3 keV
- Lower bound on sterile neutrino mass: >5.5 keV
- Constraints derived at 90% confidence level

## 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 $m_X > 1.3$ keV, or $m_s > 5.5$ keV for the case of sterile neutrinos non-resonantly produced in the early Universe, both at 90\% confidence level.