Beyond Mergers: the Role of Metallicity in Forming a Reservoir of Massive and Detached Binary Black Hole Systems

Abstract

Isolated binary evolution is a key pathway to forming binary black hole (BBH) mergers. However, the observed gravitational-wave population indicates it may not account for all mergers, especially those with high primary black hole mass ($\gtrsim 30 M_{\odot}$). Using the binary population synthesis code \texttt{POSYDON}, we simulate populations of binary systems at eight metallicities from $10^{-4}$ to $2\,Z_\odot$ and classify each BH-forming binary as a merging BBH, a detached BBH, or a system disrupted during BH formation. We find that metallicity shapes these populations differently, especially for high BH masses ($\gtrsim 25 M_{\odot}$). At lower metallicities, between $10^{-2}$ and $0.45\,Z_\odot$, massive BHs continue to form but become increasingly unlikely to merge: the number of massive merging BBHs declines sharply, while the detached population remains abundant. For $25$--$35\,M_\odot$ BHs, the detached-to-merging ratio increases by more than two orders of magnitude across this metallicity range. After weighting by cosmic star formation, massive BHs are found predominantly in detached binaries with orbital periods $\gtrsim 10^2$ days. Therefore, isolated binary evolution can form massive BBHs even if it rarely produces massive BBH mergers, leaving behind a massive and detached population that may be driven to merger through dynamical interactions in dense stellar environments.