2025MNRAS.539...45S

Star clusters can interact and merge in galactic discs, haloes, or centres. We present direct N-body simulations of binary mergers of star clusters with $M_{\star } = 2.7 \times 10^4 \, \mathrm{M_{\odot }}$ each, using the N-body code BIFROST with subsystem regularization and post-Newtonian dynamics. We include 500 $\mathrm{M_{\odot }}$ massive black holes (MBHs) in the progenitors to investigate their impact on remnant evolution. The MBHs form hard binaries interacting with stars and stellar black holes (BHs). A few Myr after the cluster merger, this produces sizable populations of runaway stars ($\sim$800 with $v_{\mathrm{ej}} \gtrsim 50 \, \mathrm{km\, s^{-1}}$) and stellar BHs ($\sim$30) escaping within 100 Myr. The remnants lose $\sim 30{{\ \rm per\ cent}}$ of their BH population and $\sim 3{{\ \rm per\ cent}}$ of their stars, with $\sim$30 stars accelerated to high velocities $\gtrsim 300 \, \mathrm{km\, s^{-1}}$. Comparison simulations of isolated clusters with central hard MBH binaries and cluster mergers without MBHs show that the process is driven by MBH binaries, while those with a single 1000 $\mathrm{M_{\odot }}$ MBH in isolated or merging clusters produce fewer runaway stars at lower velocities. Low-eccentricity merger orbits yield rotating remnants ($v_{\mathrm{rot}} \sim 3 \, \mathrm{km\, s^{-1}}$), but probing the presence of MBHs via kinematics alone remains challenging. We expect the binary MBHs to merge within a Hubble time, producing observable gravitational-wave (GW) events detectable by future GW detectors such as the Einstein Telescope and Laser Interferometer Space Antenna. The results suggest that interactions with low-mass MBH binaries formed in merging star clusters are an important additional channel for producing runaway and high-velocity stars, free-floating stellar BHs, and compact objects.