2026ApJ..1000...88L

We perform a joint analysis of the Milky Way (MW) and Andromeda (M31) satellite populations to constrain the properties of fuzzy dark matter (FDM) and thermal-relic warm dark matter (WDM). We combine MW satellite observations from the Dark Energy Survey and Pan-STARRS1 with M31 satellite data from the Pan-Andromeda Archaeological Survey, and model the corresponding observable satellite populations using the empirical galaxy-halo connection model described in Nadler et al. (2020) together with the appropriate selection functions. Uncertainties in the virial masses of the MW and M31 are incorporated through host-mass priors that linearly scale the relevant model parameters, allowing us to infer the full posterior distributions of all parameters. For the FDM case, we obtain m_FDM > 1.74 × 10^–20 eV (95% CL) and m_FDM > 1.42 × 10^–20 eV (20:1 posterior ratio). For thermal-relic WDM, we find m_WDM > 6.20 keV (95% CL) and m_WDM > 5.75 keV (20:1 posterior ratio). These results represent a moderate improvement over MW-only constraints and provide the strongest constraints to date on the FDM and WDM derived from satellite galaxy populations in the Local Group.