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 ($\lesssim10^5M_\odot$) 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 independent 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 ($\lesssim1$ 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 $10^4M_\odot$, we expect ~2-3 S-BH to end up in the stream. GC models with metallicities similar to Gaia BH3 ($\lesssim1\%$ solar) include S-BH binaries with similar BH…
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