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 +noncold ) dark matter scenarios that are characterized by the noncold DM fraction (f_nCDM) and particle mass (m_nCDM). As noncold DM species, we investigate both a fermionic (sterile neutrino) and a bosonic (ultralight 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 nonzero star formation in minihaloes (i.e., haloes with mass below 10⁸ M_⊙ ), we obtain stringent constraints on both m_nCDM and f_nCDM that go well beyond current limits. Regarding the special case of f_nCDM∼1 , for example, we find a constraint of m_nCDM>15 keV (thermal mass) for fermionic DM and m_nCDM>2 ×10^-20 eV for bosonic DM. For the opposite case of dominating cold DM, we find that at most 1% of the total DM abundance can be made of a hot fermionic or bosonic relic. All constraints are provided at the 95% confidence level.