Can the Dark Matter be $10^6$ Solar Mass Objects?

Abstract

If the dark matter in galactic halos is made up of compact, macroscopic objects (MO), such as black holes with $M_\MO >>M_{stars}$, gravitational scattering will lead to kinematic heating of the stars. Observational constraints on the amount of heating in the disk of the Milky Way put upper limits on $M_\MO \ltorder 10^{6.3}\msun $. We find limits that are three orders of magnitude more stringent by examining the heating limits in low mass stellar systems, where higher densities of dark matter and lower relative velocities disperse the stars in less than a billion years. Specifically, the structure and kinematics of the Local Group member GR8 yield a limit of $M_\MO \ltorder 6\times 10^{3}\msun$. If the properties of the dark matter are universal these results preclude the dominance of dark matter constituents in the cosmologically interesting mass range $\sim 10^6\msun$ and limit them to $M_\MO \ltorder 10^{3.7}\msun$. These results also rule out massive compact halo objects as significant contributors to the kinematic heating of the Galactic disk.