Merging black holes in young star clusters

Ugo N. Di Carlo, Nicola Giacobbo, Michela Mapelli, Mario Pasquato, Mario Spera, Long Wang, Francesco Haardt

    Research output: Contribution to journalArticlepeer-review

    Abstract

    Searching for distinctive signatures, which characterize different formation channels of binary black holes (BBHs), is a crucial step towards the interpretation of current and future gravitationalwave detections.Here, we investigate the demography ofmerging BBHs in young star clusters (SCs), which are the nursery of massive stars. We performed 4 × 103 N-body simulations of SCs with metallicity Z = 0.002, initial binary fraction 0.4, and fractal initial conditions, to mimic the clumpiness of star-forming regions. Our simulations include a novel population-synthesis approach based on the code MOBSE. We find that SC dynamics does not affect the merger rate significantly, but leaves a strong fingerprint on the properties of merging BBHs. More than 50 per cent of merging BBHs in young SCs form by dynamical exchanges in the first few Myr. Dynamically formed merging BBHs are significantly heavier than merging BBHs in isolated binaries: merging BBHs with total mass up to ~120M form in young SCs, while the maximum total mass of merging BBHs in isolated binaries with the same metallicity is only ~70 M. Merging BBHs born via dynamical exchanges tend to have smaller mass ratios than BBHs in isolated binaries. Furthermore, SC dynamics speeds up the merger: the delay time between star formation and coalescence is significantly shorter in young SCs. In our simulations, massive systems such as GW170729 form only via dynamical exchanges. Finally ~2 per cent of merging BBHs in young SCs have mass in the pair-instability mass gap (~60-120 M). This represents a unique fingerprint of merging BBHs in SCs.

    Original languageEnglish (US)
    Pages (from-to)2947-2960
    Number of pages14
    JournalMonthly Notices of the Royal Astronomical Society
    Volume487
    Issue number2
    DOIs
    StatePublished - Aug 1 2019

    Keywords

    • Binaries: general
    • Black hole physics
    • Galaxies: star clusters: general
    • Gravitational waves
    • Methods: numerical
    • Stars: kinematics and dynamics

    ASJC Scopus subject areas

    • Astronomy and Astrophysics
    • Space and Planetary Science

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