Imprints of fermionic and bosonic mixed dark matter on the 21-cm signal at cosmic dawn

The 21-cm signal from the epoch of cosmic dawn prior to reionization consists of a promising observable to gain new insights into the dark matter (DM) sector. In this paper, we investigate its potential to constrain mixed (cold + non-cold) dark matter scenarios that are characterised by the non-cold DM fraction (f ₍₂₃₌) and particle mass (m ₍₂₃₌). As non-cold DM species, we investigate both a fermionic (sterile neutrino) and a bosonic (ultra-light axion) particle. We show how these scenarios affect the global signal and the power spectrum using a halo-model implementation of the 21-cm signal at cosmic dawn. Next to this study, we perform an inference-based forecast study based on realistic mock power spectra from the Square Kilometre Array (SKA) telescope. Assuming inefficient, yet non-zero star-formation in minihaloes (i. e. haloes with mass below 10⁸ M_), we obtain stringent constraints on both m ₍₂₃₌ and f ₍₂₃₌ that go well beyond current limits. Regarding the special case of f ₍₂₃₌ 1, for example, we find a constraint of m ₍₂₃₌>15 keV (thermal mass) for fermionic DM and m ₍₂₃₌>210^-20 eV for bosonic DM. For the opposite case of dominating cold DM, we find that at most one percent of the total DM abundance can be made of a hot fermionic or bosonic relic. All constraints are provided at the 95 percent confidence level.