Dynamical evolution of young star clusters, runaway collisions, and massive black hole formation

The radial surface brightness profiles of old star clusters, such as globular clusters, are typically fitted with the well-known King profile. However, such profiles do not appear to represent the young star clusters nearly as well. We study the evolution of the density profile of star clusters from an Elson in young massive clusters to a King profile in the later evolutionary stages. A compact star cluster with a flatter density profile has a higher dynamical interaction rate among its stars compared to a typical King profile. These dense young clusters can undergo collisional runaway mergers of stars forming intermediate-mass black holes (IMBHs), which cannot form through the stellar evolution of standalone stars. We present a suite of N-body simulations performed using the CMC cluster dynamics code across a grid of initial conditions in cluster metallicity, physical sizes, masses, and initial density profiles. We find that low metallicity, compact massive clusters are more efficient at forming IMBHs with masses M>200Μ_?. We will discuss the specific regions of parameter space that facilitate the formation of these massive black holes and their implications for LIGO/Virgo.