Closing the window on fuzzy dark matter with the 21-cm signal

Fuzzy dark matter (FDM) is a well-motivated candidate for dark matter (DM) as its tiny mass and large de-Broglie wavelength suppress small-scale matter fluctuations, thereby solving some of the small-scale discrepancies in ΛCDM. Although it has been ruled out as the single component of DM by several observables, there is still a region in the FDM parameter space (the "FDM window,"10-25 eV≪mFDM≪10-23 eV) where FDM is allowed to comprise a large portion of the total DM. In this work, for the first time, we study the signature of FDM (comprised of ultralight axions) in fractions less than unity on the 21-cm signal and its detectability by 21-cm interferometers such as HERA, taking into account the degeneracy with both astrophysical and cosmological parameters, using a new pipeline that combines modified versions of the camb and 21-cmfast codes. Our forecasts imply that HERA in its design performance will be sensitive to FDM fractions as small as 1% in the FDM window, and improve over existing bounds for other masses by up to an order of magnitude.