Robust bounds on MACHOs from the faintest galaxies

Abstract: We use the dynamical heating of stars in ultrafaint dwarf (UFD) galaxies to set limits on Massive Compact Halo Objects (MACHOs). In our analysis we study the robustness of the bounds under uncertainties in key UFD parameters, such as the half-light radius, stellar velocity dispersion, total halo mass and dark matter and stellar density profiles. We apply this framework to both well-established UFD candidates, as well as the recently discovered UFD candidate Ursa Major III/UNIONS 1. We find that multiple UFDs yield consistently strong limits in the mass range $10\, M_\odot \lesssim M_{\rm MACHO} \lesssim 10^9\, M_\odot$, underscoring the robustness of a previous analysis solely based on Segue I. We also demonstrate that Ursa Major III, if confirmed as an UFD, would improve the constraints significantly, providing the strongest constraints on MACHO dark matter in the mass range $1\, M_{\odot}\lesssim M_{\rm MACHO} \lesssim 10^5\, M_\odot$.