Fuzzy Dark Matter at Cosmic Dawn: New 21-cm Constraints

Abstract: Potential small-scale discrepancies in the picture of galaxy formation painted by the $\Lambda$CDM paradigm have led to considerations of modified dark matter models. One such dark matter model that has recently attracted much attention is fuzzy dark matter (FDM). In FDM models, the dark matter is envisaged to be an ultra-light scalar field with a particle mass $m_{\rm FDM} \sim 10^{-22} $ eV. This yields astronomically large de Broglie wavelengths which can suppress small-scale structure formation and give rise to the observed kpc-sized density cores in dwarf galaxies. We investigate the evolution of the 21-cm signal during Cosmic Dawn and the Epoch of Reionization (EoR) in $\Lambda$FDM cosmologies using analytical models. The delay in source formation and the absence of small halos in $\Lambda$FDM significantly postpone the Ly$\alpha$ coupling, heating, as well as the reionization of the neutral hydrogen of the intergalactic medium. As a result, the absorption feature in the evolution of the global 21-cm signal has a significantly smaller full width at half maximum ($\Delta z \lesssim 3$), than $\Lambda$CDM ($\Delta z \simeq 6$). This alone rules out $m_{\rm FDM} < 6 \times 10^{-22}$ eV as a result of the $2\sigma$ lower limit $\Delta z \gtrsim 4$ from EDGES High-Band. As a result, $\Lambda$FDM is not a viable solution to the potential small-scale problems facing $\Lambda$CDM. Finally, we show that any detection of the 21-cm signal at redshifts $z > 14$ by interferometers such as the SKA can also exclude $\Lambda$FDM models.