Cold Dark Matter’s Small‐Scale Crisis Grows Up

Abstract

The Cold Dark Matter (CDM) theory predicts a wealth of substructure within dark halos. These predictions match observations of galaxy clusters like the nearby Virgo cluster. However, CDM has a "small scale crisis" since galaxies dominate the halo with little substructure while the model predicts that galaxies should be scaled versions of galaxy clusters with abundant substructure. Compared to CDM predictions, the Milky Way and Andromeda are "missing" objects with velocity dispersions $\sigma \geq 10$ km s$^{-1}$. The energy scale of these missing satellites is low enough that stellar winds and supernovae might remove gas and suppress the formation of their luminous stellar components. Here, we show that the small scale crisis persists in fossil groups that have masses of up to 40% of the nearby Virgo cluster of galaxies. Fossil groups are missing satellites with luminosities that occur at the predicted frequency in the Virgo cluster. Moreover, the "missing galaxies" in fossil groups are nearly as luminous as the Milky Way with a velocity dispersion $\sigma \leq$ 150 km s$^{-1}$.