Dynamical formation of Gaia BH3 in the progenitor globular cluster of the ED-2 stream

Abstract

Context. The star–black hole (S–BH) binary known as Gaia BH3, discovered by the Gaia Collaboration is chemically and kinemati-cally associated with the metal-poor ED-2 stream in the Milky Way halo.Aims. We explore the possibility that Gaia BH3 was assembled dynamically in the progenitor globular cluster (GC) of the ED-2stream.Methods. We used a public suite of star-by-star dynamical Monte Carlo models to identify S–BH binaries in GCs with different initialmasses and (half-mass) radii.Results. We show that a likely progenitor of the ED-2 stream was a relatively low-mass (.105 M ) GC with an initial half-massradius of ∼4 pc. Such a GC can dynamically retain a large fraction of its BH population and dissolve on the orbit of ED-2. Fromthe suite of models we find that GCs produce ∼3−30 S–BH binaries, approximately independently of initial GC mass and inverselycorrelated with initial cluster radius. Scaling the results to the Milky Way GC population, we find that ∼75% of the S–BH binariesformed in GCs are ejected from their host GC, all in the early phases of evolution (.1 Gyr); these are expected to no longer be closeto streams. The ∼25% of S–BH binaries retained until dissolution are expected to form part of streams, such that for an initial massof the progenitor of ED-2 of a few 104 M , we expect ∼2−3 S–BH to end up in the stream. GC models with metallicities similar toGaia BH3 (.1% solar) include S–BH binaries with similar BH masses (&30 M ), orbital periods, and eccentricities.Conclusion. We predict that the Galactic halo contains of order 105 S–BH binaries that formed dynamically in GCs, a fraction ofwhich may readily be detected in Gaia DR4. The detection of these sources provides valuable tests of BH dynamics in clusters andtheir contribution to gravitational wave sources