Abstract
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 ($k\sim0.1\,Mpc^-1$)
modulation, by dark-matter--baryon relative velocities, of the small-scale
($k10-10^3\, Mpc^-1$) 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 $m_\alpha\approx
10^-18eV$, two decades of mass higher than current constraints.
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