Milky Way satellite velocities reveal the dark matter power spectrum at small scales

Dark matter (DM) properties at small scales remain uncertain. Recent theoretical and observational advances have provided the tools to narrow them down. Here, we show for the first time that the correlation between internal velocities and sizes of dwarf galaxies is a sharp probe of small-scale DM properties. We study modified DM power spectra, motivated by DM production during inflation. Using semianalytic models and scaling relations, we show that such models can change the kinematics and structure of dwarf galaxies without strongly affecting their total abundance. We analyze data from Milky Way classical satellite galaxies and those discovered with the Sloan Digital Sky Survey, finding that the DM power spectrum at comoving scales 4 Mpc-1<k<37 Mpc-1 cannot deviate by more than a factor of ∼2.5 from scale invariance. Our results are robust against baryonic uncertainties such as the stellar mass-halo mass relation, halo occupation fraction, and subhalo tidal disruption; allowing us to independently constrain them. This work thus opens a window to probe both dwarf galaxy formation models and small-scale DM properties.