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 kinematically 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-2 stream. Methods. We used a public suite of star-by-star dynamical Monte Carlo models to identify S–BH binaries in GCs with different initial masses 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-mass radius of ∼4 pc. Such a GC can dynamically retain a large fraction of its BH population and dissolve on the orbit of ED-2. From the suite of models we find that GCs produce ∼3 − 30 S–BH binaries, approximately independently of initial GC mass and inversely correlated with initial cluster radius. Scaling the results to the Milky Way GC population, we find that ∼75% of the S–BH binaries formed in GCs are ejected from their host GC, all in the early phases of evolution (≲1 Gyr); these are expected to no longer be close to streams. The ∼25% of S–BH binaries retained until dissolution are expected to form part of streams, such that for an initial mass of 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 to Gaia 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 of which may readily be detected in Gaia DR4. The detection of these sources provides valuable tests of BH dynamics in clusters and their contribution to gravitational wave sources.