Dynamical formation of Gaia BH3 in the progenitor globular cluster of the ED-2 stream
arxiv(2024)
摘要
Context. The star-black hole (S-BH) binary discovered by the Gaia
Collaboration - Gaia BH3 - 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 use a public suite of star-by-star dynamical Monte Carlo models
by Kremer et al. (2020) 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 (≲10^5M_⊙) GC with an initial half-mass radius of 2 - 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 in the early phases of evolution
(≲1 Gyr); these are expected to no longer be close to the stream. 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_⊙, 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 (≳30M_⊙), orbital periods, and
eccentricities.
Conclusions. We predict the Galactic halo contains of order 10^5 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 the possible role in formation of gravitational
wave sources.
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