Star clusters can interact and merge in galactic discs, haloes, or centres. We present direct N -body simulations of binary mergers of star
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. [ABSTRACT FROM AUTHOR]
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