Probing ultra-light axions with the 21-cm Signal during Cosmic Dawn

Ultra-light axions (ULAs) are a promising and intriguing set of dark-matter candidates. We study the prospects to use forthcoming measurements of 21-cm fluctuations from cosmic dawn to probe ULAs. We focus in particular on the velocity acoustic oscillations (VAOs) in the large-scale 21-cm power spectrum, features imprinted by the long-wavelength ($k\sim 0.1{\mathrm{M}\mathrm{p}\mathrm{c}}^{-1}$) modulation, by dark-matter–baryon relative velocities, of the small-scale ($k\sim 10-{10}^3{\mathrm{M}\mathrm{p}\mathrm{c}}^{-1}$) power required to produce the stars that heat the neutral hydrogen. Damping of small-scale power by ULAs reduces the star-formation rate at cosmic dawn which then leads to a reduced VAO amplitude. Accounting for different assumptions for feedback and foregrounds, experiments like HERA may be sensitive to ULAs with masses up to $m_{\alpha}\approx 10^{-18}$\text{eV}$$, two decades of mass higher than current constraints.