Mixed Dark Matter: Limits from the Milky Way Satellite Galaxies
The Standard Model of particle physics contains a diverse set of particle species, motivating the possibility of a similarly complex dark sector. Here we study two-component dark matter (DM) mixtures, in which one component behaves as standard CDM while the other suppresses the formation of small-scale structure, either through an astrophysically relevant de~Broglie wavelength (fuzzy DM; FDM) or collisional damping from temperature-independent scattering (interacting DM; IDM). Using the observed population of Milky Way satellite galaxies, we derive new leading constraints on the parameter spaces of mixed FDM and of mixed IDM coupled to photons ($\gamma$-DM), neutrinos ($\nu$-DM), or baryons ($p$-DM), for beyond-CDM fractions down to $50\%$. We require that the linear matter power spectra of allowed models remain less suppressed than a constrained reference model. The resulting $95\%$ confidence bounds on FDM mass and IDM cross section weaken systematically with decreasing fraction, following distinct power-law scalings. At $50\%$ fraction, IDM cross section bounds weaken by a factor of $\sim$2--6 and FDM mass bounds by $\sim$1.5, relative to the $100\%$ case. We forecast that idealized future satellite surveys, which adopt approximate LSST sensitivity thresholds, can improve these $100\%$ bounds by a factor of $\sim$1.6--14 for IDM and $\sim$3 for FDM. Self-consistent cosmological simulations of mixed DM scenarios will be essential to more robustly characterize the degeneracy between particle physics parameters and fractional contribution, to extend constraints to lower fractions, and to identify signatures beyond satellite abundance to further inform these models.