Massive black hole binaries from runaway collisions: the impact of metallicity

TL;DR

This study uses advanced N-body simulations to explore how metallicity influences the formation of massive black holes through runaway collisions in star clusters, highlighting the potential for IMBH formation at low metallicity.

Contribution

It introduces upgraded stellar wind and supernova models in N-body simulations to analyze metallicity effects on black hole formation in star clusters.

Findings

Higher remnant masses (~250 Msun) at low metallicity.

Most long-lived binaries with collision products are black hole binaries.

Significant fraction (~0.6) of collision products remain bound in clusters.

Abstract

The runaway collision scenario is one of the most promising mechanisms to explain the formation of intermediate-mass black holes (IMBHs) in young dense star clusters. On the other hand, the massive stars that participate in the runaway collisions lose mass by stellar winds. In this paper, we discuss new N-body simulations of massive (6.5x10^4 Msun) star clusters, in which we added upgraded recipes for stellar winds and supernova explosion at different metallicity. We follow the evolution of the principal collision product (PCP), through dynamics and stellar evolution, till it forms a stellar remnant. At solar metallicity, the mass of the final merger product spans from few solar masses up to ~30 Msun. At low metallicity (0.01-0.1 Zsun) the maximum remnant mass is ~250 Msun, in the range of IMBHs. A large fraction (~0.6) of the PCPs are not ejected from the parent star cluster and acquire stellar or black hole (BH) companions. Most of the long-lived binaries hosting a PCP are BH-BH binaries. We discuss the importance of this result for gravitational wave detection.