Excluding fuzzy dark matter with sizes and stellar kinematics of ultrafaint dwarf galaxies

We use observations of ultrafaint dwarf (UFD) galaxies to constrain the particle mass of ultralight dark matter. Potential fluctuations created by wave interference in virialized "fuzzy " dark matter (FDM) halos dynamically heat stellar orbits in UFDs, some of which exhibit velocity dispersions of ? 3 km/s and sizes ?40 pc. Using simulations of FDM halos, and existing measurements of sizes and stellar radial velocities in Segue 1 and Segue 2 UFDs, we derive a lower limit on the dark matter particle mass of m(FDM )> 3 x 10(-19) eV at 99% confidence, marginalized over host halo circular velocity. This constraint is conservative as it is derived under the assumption that soliton heating is negligible, and that no other sources of non-FDM dynamical heating of stars operate to increase velocity dispersion. It can potentially be strengthened by future spectroscopic observations of additional stars in ultrafaint galaxies and by tightening theoretical constraints on the soliton size-halo mass relation. However, even the current conservative lower limit on the ultralight dark matter mass makes it indistinguishable from cold dark matter at the scales probed by existing astronomical observations.