arXiv subjects

Kyle Kremer

Publications and source records attributed to Kyle Kremer.

At least 37 records · Page 2Linked to original sources

arXiv2026

A Path to an All-Sky Survey with Roman

A deep, space-based, all-sky near-infrared survey carried out with the Nancy Grace Roman Space Telescope would constitute a foundational astronomical infrastructure for decades to come. In this white paper, we present a concrete and feasible path to imaging the entire sky at $\sim0.1''$ resolution, beginning with high-impact fields in Cycle 1 and scaling to ultra-wide coverage within the nominal mission. This first-epoch survey will reach $\mathrm{H}\sim25.5$ AB mag (5$σ$) and maximize synergies with contemporaneous observatories, while preserving substantial time for other ambitious Roman programs. We outline representative scheduling scenarios and an example Cycle 1 program that triples early Roman-LSST overlap and delivers high-value community data products such as LSST forced photometry, joint \textit{Gaia}-Roman astrometry, and catalogs of Galactic substructure, stong lenses, and other rare systems. The Cycle 1 program will lay the foundation for an eventual all-sky survey, while also delivering high-impact early science. We invite broad community participation in shaping and carrying out both the initial program and the long-term vision of an all-sky Roman survey.

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arXiv2026

Discovery of a double white dwarf in the Galactic globular cluster NGC 6397

Binaries in the cores of globular clusters are known to prevent the gravitational collapse of the cluster, and simulations predict that the core of NGC 6397 contains a large number of white dwarfs (WDs), of which many are expected to be part of a binary system. In this work, we report the discovery of a compact binary system consisting of two WDs in the centre of the Galactic globular cluster NGC 6397. The system, known in the literature as NF1, was observed as part of a MUSE radial-velocity survey aiming at characterizing the binary population in the centre of NGC 6397. The spectral analysis of NF1 provides an effective temperature of 16000 K and a surface gravity (log g) of 5.72 (cgs), which is consistent with an extremely low-mass He-core WD nature. This is further supported by the mass of 0.23 +/- 0.03 Msun obtained from fitting the star's spectral energy distribution using its HST magnitude in various filters. The system has a circular orbit with a period of 0.54 days. The radial velocities show a large semi-amplitude of 200 km/s, implying a minimum mass of 0.78 Msun for the invisible companion, which is likely another WD, or a neutron star if the inclination of the system is smaller than about 50 deg. Some significant residuals in radial velocity remain with our best orbital solution and we tested whether a model with a third body can explain these deviations. While this possibility seems promising, additional measurements are needed to confirm whether the star is actually part of a triple system.

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arXiv2026

Shadows of the Colossus: Hierarchical Black Hole Mergers in a 10-million-body Globular Cluster Simulation

The LIGO/Virgo/Kagra (LVK) Collaboration has detected numerous binary black hole mergers with properties that challenge standard binary evolution scenarios, such as component masses above the pair-instability gap and high spin magnitudes. Dense stellar environments such as globular clusters provide a natural channel for producing such systems through hierarchical mergers, where black hole remnants formed in earlier mergers are retained in the cluster and undergo successive mergers. However, gravitational-wave recoil kicks often eject merger remnants from typical globular clusters, which limits hierarchical growth. Massive clusters with deeper potential wells, such as those found in giant elliptical galaxies like M87, may overcome this barrier, but direct simulations of such massive globular clusters remains computationally challenging. In this study, we present a 10-million-body cluster simulation performed with the $\texttt{Cluster Monte Carlo}$ ($\texttt{CMC}$) code, referred to as $\texttt{colossus}$, which serves as a proxy for the most massive low-metallicity globular clusters observed in the local Universe. This simulation demonstrates that extended chains of hierarchical mergers can occur in massive globular clusters, producing black holes up to fifth generation with masses approaching $250\,M_\odot$, comparable to the most massive LVK events observed to date (e.g., GW231123). Combining the $\texttt{colossus}$ simulation with the previous $\texttt{CMC Cluster Catalog}$, we develop a framework to extrapolate binary black hole merger predictions for the thousands of globular clusters seen in the Virgo Supercluster.

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arXiv2025

Beyond Hierarchical Mergers: Accretion-Driven Origins of Massive, Highly Spinning Black Holes in Dense Star Clusters

GW231123, the most massive binary black hole (BBH) merger detected by LIGO/Virgo/KAGRA, highlights the need to understand the origins of massive, high-spin stellar black holes (BHs). Dense star clusters provide natural environments for forming such systems, beyond the limits of standard massive star evolution to core collapse. While repeated BBH mergers can grow BHs through dynamical interactions (the so-called "hierarchical merger" channel), most star clusters with masses $\lesssim 10^6\,M_\odot$ have escape speeds too low to retain higher-generation BHs, limiting growth into or beyond the mass gap. In contrast, BH--star collisions with subsequent accretion of the collision debris can grow and retain BHs irrespective of the cluster escape speed. Using $N$-body (Cluster Monte Carlo) simulations, we study BH growth and spin evolution through this process and we find that accretion can drive BH masses up to at least $\sim200\,M_\odot$, with spins set by the details of the growth history. BHs up to about $150\,M_\odot$ can reach dimensionless spins $χ\gtrsim 0.7$ via single coherent episodes, while more massive BHs form through multiple stochastic accretion events and eventually spin down to $χ\lesssim 0.4$. These BHs later form binaries through dynamical encounters, producing BBH mergers that contribute up to $\sim10\%$ of all detectable events, comparable to predictions for the hierarchical channel. However, the two pathways predict distinct signatures: hierarchical mergers yield more unequal mass ratios, whereas accretion-grown BHs preferentially form near-equal-mass binaries. The accretion-driven channel allows dense clusters with low escape speeds, such as globular clusters, to produce highly spinning BBHs with both components in or above the mass gap, providing a natural formation pathway to GW231123-like systems.

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arXiv2025

Hierarchical Triples vs. Globular Clusters: Binary black hole merger eccentricity distributions compete and evolve with redshift

The formation mechanisms of merging binary black holes (BBHs) observed by the LIGO-Virgo-KAGRA collaboration remain uncertain. Detectable eccentricity provides a powerful diagnostic for distinguishing between different formation channels, but resolving their eccentricity distributions requires the detection of a large number of eccentric mergers. Future gravitational wave detectors such as the Einstein Telescope and Cosmic Explorer will detect tens of thousands of BBH mergers out to redshifts $z \ge 10$, making it critical to understand the redshift-dependent evolution of eccentricity distributions. We simulate this evolution for two key channels: dynamical assembly in globular clusters (GCs), which leads to rapid, eccentric mergers; and hierarchical triples in the field, where three-body dynamics can induce eccentricity in the inner binary. When considering all BBH mergers, the GC channel dominates overall, consistent with previous studies. However, when focusing on mergers with detectable eccentricity in next-generation detectors, we find that hierarchical triples dominate the eccentric merger rate at $0\le z \le 4$, with GC mergers becoming competitive at higher redshifts. Across all model variations, eccentric mergers in the local Universe ($z\lesssim 1$) have significant contributions from field triples, challenging the common view that such systems primarily form in dense environments. We show that, regardless of cluster and stellar evolution uncertainties, hierarchical triples contribute at least 30 per cent of eccentric mergers across a large range of redshifts.

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arXiv2025

Connecting Cores and Black Hole Dynamics Across Scales: From Globular Clusters to Massive Ellipticals

The centers of massive elliptical galaxies exhibit a wide range in density profiles, from central cusps to resolved cores with order kiloparsec sizes. The cored ellipticals have been linked to the presence of supermassive black hole binaries that excavate their hosts' central stellar populations through three-body encounters. This connection between cores and black holes similarly operates in globular clusters, which also exhibit a bimodality in cored and core-collapsed architectures, respectively rich and depleted in stellar black holes. We report new estimates of the total black hole mass in 25 Galactic globular clusters based on a suite of roughly 150 Monte Carlo $N$-body simulations that fit observed surface brightness and velocity dispersion profiles. We show that both globular clusters and massive elliptical galaxies individually exhibit strong correlations between total black hole mass ($M_\bullet$) and core radius ($r_c$), and that these individual relations share a common power-law exponent to within $1σ$ statistical precision: $M_\bullet \sim r_c^{1.3}$. The individual relations appear to be offset, suggesting swarms of stellar black holes scour globular cluster cores more efficiently than lone supermassive black holes scour the cores of massive ellipticals. Yet the shared basis of core scouring via black hole binaries hints at a unified $M_{\bullet}-r_c$ connection across over 10 orders of magnitude in $M_\bullet$. Our findings imply core radius measurements may offer a powerful observational constraint on black hole merger rates, from kilohertz sources detectable by LIGO/Virgo/KAGRA formed in globular clusters to millihertz and nanohertz sources formed in massive elliptical galaxies.

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arXiv2025

Observing compact Pop III star clusters and the presence of cosmic streaming

The formation of the Universe's first luminous stellar structures depends on the unique conditions at "Cosmic Dawn," which are set by the underlying cosmological model and early baryonic physics. Observations suggest that high-$z$ star clusters reached stellar surface densities above $10^5 M_\odot$ pc$^{-2}$, suggesting scenarios where models predict that the ability of stellar feedback to counter gravitational collapse is severely limited. We investigate the first star clusters in a suite of AREPO simulations, which explore the capacity for $Λ$CDM halos to maximally form high-density systems without feedback. We include the effects of the supersonic baryon-dark matter streaming velocity, an effect that impacts gas density and distribution in early minihalos. We show that early star clusters can reach high densities even in regions of strong supersonic streaming, provided feedback is weak. We analyze the interplay of the stream velocity and the dynamical processes of structure formation, finding that JWST has the opportunity to detect the brightest, most massive objects in our computational box. The detection of individual $z\geq12$ Pop III star clusters below $10^7M_\odot$ is challenging, although lensing could reveal these objects in rare configurations, especially if a top-heavy IMF is present. We find that accounting for baryonic clusters separately from dark matter halos complicates predictions for the faint-end of the high-$z$ UVLF, with competing effects from the stream velocity and low-mass clusters outside of halos. Finally, we explore clustering of star clusters as a promising probe of the stream velocity in these systems.

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arXiv2025

Compact Objects in Globular Clusters

It is now widely established that globular clusters host robust populations of white dwarfs, neutron stars, and black holes throughout their lifetimes. Within clusters, dynamical processes enabled by stellar densities thousands to millions of times larger than typical galactic environments facilitate interactions involving these stellar remnants that give rise to an array of astrophysical phenomena. In particular, stellar clusters have emerged as an important formation site for X-ray sources, radio pulsars, and merging black hole binaries similar to those recently detected as gravitational wave sources by the LIGO/Virgo/KAGRA detectors. This article reviews our current understanding of compact objects in globular clusters, discussing current observational evidence, ways these objects influence the dynamical evolution of their hosts, and future prospects.

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arXiv2025

Can slow pulsars in Milky Way globular clusters form via partial recycling?

Alongside the population of several hundred radio millisecond pulsars currently known in Milky Way globular clusters, a subset of six slowly spinning pulsars (spin periods $0.3-4\,$s) are also observed. With inferred magnetic fields $\gtrsim 10^{11}\,$G and characteristic ages $\lesssim10^8\,$yr, explaining the formation of these apparently young pulsars in old stellar populations poses a major challenge. One popular explanation is that these objects are not actually young but instead have been partially spun up via accretion from a binary companion. In this scenario, accretion in a typical low-mass X-ray binary is interrupted by a dynamical encounter with a neighboring object in the cluster. Instead of complete spin up to millisecond spin periods, the accretion is halted prematurely, leaving behind a ''partially recycled'' neutron star. In this Letter, we use a combination of analytic arguments motivated by low-mass X-ray binary evolution and $N$-body simulations to show that this partial-recycling mechanism is not viable. Realistic globular clusters are not sufficiently dense to interrupt mass transfer on the short timescales required to achieve such slow spin periods. We argue that collapse of massive white dwarfs and/or neutron star collisions are more promising ways to form slow pulsars in old globular clusters.

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arXiv2025

Localizing Dynamically-Formed Black Hole Binaries in Milky Way Globular Clusters with LISA

The dynamical formation of binary black holes (BBHs) in globular clusters (GCs) may contribute significantly to the observed gravitational wave (GW) merger rate. Furthermore, LISA may detect many BBH sources from GCs at mHz frequencies, enabling the characterization of such systems within the Milky Way and nearby Universe. In this work, we use Monte Carlo N-body simulations to construct a realistic sample of Galactic clusters, thus estimating the population, detectability, and parameter measurement accuracy of BBHs hosted within them. In particular, we show that the GW signal from $0.7\pm 0.7$, $2.0\pm 1.7$, $3.6\pm 2.3$, $13.4\pm 4.7$ BBHs in Milky Way GCs can exceed the signal-to-noise ratio threshold of $\rm SNR =30$, 5, 3, and 1 for a 10-year LISA observation, with $\sim 50\%$ of detectable sources exhibiting high eccentricities ($e\gtrsim0.9$). Moreover, the Fisher matrix and Bayesian analyses of the GW signals indicate these systems typically feature highly-resolved orbital frequencies ($δf_{\rm orb}/ f_{\rm orb} \sim 10^{-7}-10^{-5}$) and eccentricities ($δe/ e \sim 10^{-3}-0.1$), as well as a measurable total mass when SNR exceeds $\sim20$. Notably, we show that high-SNR BBHs can be confidently localized to specific Milky Way GCs with a sky localization accuracy of $δΩ\sim 1$~deg$^2$, and address the large uncertainties in their distance measurement ($δR \sim 0.3 - 20$~kpc). The detection and localization of even a single BBH in a Galactic GC would allow accurate tracking of its long-term orbital evolution, enable a direct test of the role of GCs in BBH formation, and provide a unique probe into the evolutionary history of Galactic clusters.

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arXiv2025

Zooming In On The Multi-Phase Structure of Magnetically-Dominated Quasar Disks: Radiation From Torus to ISCO Across Accretion Rates

Recent radiation-thermochemical-magnetohydrodynamic simulations resolved formation of quasar accretion disks from cosmological scales down to ~300 gravitational radii $R_{g}$, arguing they were 'hyper-magnetized' (plasma $β\ll1$ supported by toroidal magnetic fields) and distinct from traditional $α$-disks. We extend these, refining to $\approx 3\,R_{g}$ around a $10^{7}\,{\rm M_{\odot}}$ BH with multi-channel radiation and thermochemistry, and exploring a factor of 1000 range of accretion rates ($\dot{m}\sim0.01-20$). At smaller scales, we see the disks maintain steady accretion, thermalize and self-ionize, and radiation pressure grows in importance, but large deviations from local thermodynamic equilibrium and single-phase equations of state are always present. Trans-Alfvenic and highly-supersonic turbulence persists in all cases, and leads to efficient vertical mixing, so radiation pressure saturates at levels comparable to fluctuating magnetic and turbulent pressures even for $\dot{m}\gg1$. The disks also become radiatively inefficient in the inner regions at high $\dot{m}$. The midplane magnetic field remains primarily toroidal at large radii, but at super-Eddington $\dot{m}$ we see occasional transitions to a poloidal-field dominated state associated with outflows and flares. Large-scale magnetocentrifugal and continuum radiation-pressure-driven outflows are weak at $\dot{m}<1$, but can be strong at $\dot{m}\gtrsim1$. In all cases there is a scattering photosphere above the disk extending to $\gtrsim 1000\,R_{g}$ at large $\dot{m}$, and the disk is thick and flared owing to magnetic support (with $H/R$ nearly independent of $\dot{m}$), so the outer disk is strongly illuminated by the inner disk and most of the inner disk continuum scatters or is reprocessed at larger scales, giving apparent emission region sizes as large as $\gtrsim 10^{16}\,{\rm cm}$.

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arXiv2025

Spin-Orbit Alignment in Merging Binary Black Holes Following Collisions with Massive Stars

Merging binary black holes (BBHs) formed dynamically in dense star clusters are expected to have uncorrelated spin--orbit orientations since they are assembled through many random interactions. However, measured effective spins in BBHs detected by LIGO/Virgo/KAGRA hint at additional physical processes that may introduce anisotropy. Here we address this question by exploring the impact of stellar collisions, and accretion of collision debris, on the spin--orbit alignment in merging BBHs formed in dense star clusters. Through hydrodynamic simulations, we study the regime where the disruption of a massive star by a BBH causes the stellar debris to form individual accretion disks bound to each black hole. We show that these disks, which are randomly oriented relative to the binary orbital plane after the initial disruption of the star, can be reoriented by strong tidal torques in the binary near pericenter passages. Following accretion by the BHs on longer timescales, BBHs with small but preferentially positive effective spin parameters ($χ_{\rm eff} \lesssim 0.2$) are formed. Our results indicate that BBH collisions in young massive star clusters could contribute to the observed trend toward small positive $χ_{\rm eff}$, and we suggest that the standard assumption often made that dynamically assembled BBHs should have isotropically distributed BH spins is not always justified.

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arXiv2025

Black Hole Accretion and Spin-up Through Stellar Collisions in Dense Star Clusters

Dynamical interactions in dense star clusters could significantly influence the properties of black holes, leaving imprints on their gravitational-wave signatures. While previous studies have mostly focused on repeated black hole mergers for spin and mass growth, this work examines the impact of physical collisions and close encounters between black holes and (non-compact) stars. Using Monte Carlo N-body models of dense star clusters, we find that a large fraction of black holes retained upon formation undergo collisions with stars. Within our explored cluster models, the proportion of binary black hole mergers affected by stellar collisions ranges from $10\%$ to $60\%$. If all stellar-mass black holes are initially non-spinning, we find that up to $40\%$ of merging binary black holes may have components with dimensionless spin parameter $χ\gtrsim 0.2$ because of prior stellar collisions, while typically about $10\%$ have spins near $χ= 0.7$ from prior black hole mergers. We demonstrate that young star clusters are especially important environments as they can produce collisions of black holes with very massive stars, allowing significant spin up of the black holes through accretion. Our predictions for black hole spin distributions from these stellar collisions highlight their sensitivity to accretion efficiency, underscoring the need for detailed hydrodynamic calculations to better understand the accretion physics following these interactions.

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arXiv2024

Lower-mass-gap Black Holes in Dense Star Clusters

The existence of compact stellar remnants in the mass range $2-5\,M_{\odot}$ has long been debated. This so-called lower mass gap was initially suggested by the lack of low-mass X-ray binary observations with accretors about $2-5\,M_{\odot}$, but it has recently been called into question following newer observations, including a lower-mass-gap candidate with a millisecond pulsar companion in the dense globular cluster NGC 1851. Here we model NGC 1851 with a grid of similar dense star clusters utilizing the state-of-the-art Monte Carlo $N$-body code \texttt{CMC}, and we specifically study the formation of lower-mass-gap black holes. We demonstrate that both massive star evolution and dynamical interactions can contribute to forming lower-mass-gap black holes. In general, the collapse of massive remnants formed through mergers of neutron stars or massive white dwarfs produces the largest number of lower-mass-gap black holes among all formation channels. However, in more massive clusters, supernova core collapse can contribute comparable numbers. Our NGC 1851-like models can reproduce millisecond pulsar -- lower-mass-gap black hole binaries similar to the observed system. Additionally, the lower-mass-gap black holes can also become components of dynamically assembled binaries, and some will be in merging black hole - neutron star systems similar to the recently detected gravitational wave source GW230529. However, the corresponding merger rate is probably $\lesssim 1~{\rm Gpc^{-3}\,yr^{-1}}$.

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arXiv2024

Feedback-regulated Seed Black Hole Growth in Star-Forming Molecular Clouds and Galactic Nuclei

The detection of supermassive black holes (SMBHs) in high-redshift luminous quasars may require a phase of rapid accretion, and as a precondition, substantial gas influx toward seed black holes (BHs) from kilo-parsec or parsec scales. Our previous research demonstrated the plausibility of such gas supply for BH seeds within star-forming giant molecular clouds (GMCs) with high surface density ($\sim 10^4\,{\rm {\rm M_\odot}\, pc}^{-2}$), facilitating ``hyper-Eddington'' accretion via efficient feeding by dense clumps which are driven by turbulence and stellar feedback. This article investigates the impacts of feedback from accreting BHs on this process, including radiation, mechanical jets, and highly relativistic cosmic rays. We run a suite of numerical simulations to explore diverse parameter spaces of BH feedback, including the sub-grid accretion model, feedback energy efficiency, mass loading factor, and initial metallicity. Utilizing radiative feedback models inferred from the slim disk, we find that hyper-Eddington accretion is still achievable, yielding BH bolometric luminosities as high as $10^{41}$ -- $10^{44}\,\rm erg/s$, depending on the GMC properties and specific feedback model assumed. We find the maximum possible mass growth of seed BHs ($ΔM_{\rm BH}^{\rm max}$) is regulated by the momentum deposition rate from BH feedback, $\dot{p}_{\rm feedback}/(\dot{M}_{\rm BH} c)$, which leads to an analytic scaling that agrees well with simulations. This scenario predicts the rapid formation of $\sim 10^4\,\rm M_\odot$ intermediate-massive BHs (IMBHs) from stellar-mass BHs within $\sim \rm Myr$. Furthermore, we examine the impacts of sub-grid accretion models and how BH feedback may influence star formation within these cloud complexes.

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arXiv2024

FORGE'd in FIRE III: The IMF in Quasar Accretion Disks from STARFORGE

Recently, we demonstrated self-consistent formation of strongly-magnetized quasar accretion disks (QADs) from cosmological radiation-magnetohydrodynamic-thermochemical galaxy-star formation simulations, including the full STARFORGE physics shown previously to produce a reasonable IMF under typical ISM conditions. Here we study star formation and the stellar IMF in QADs, on scales from 100 au to 10 pc from the SMBH. We show it is critical to include physics often previously neglected, including magnetic fields, radiation, and (proto)stellar feedback. Closer to the SMBH, star formation is suppressed, but the (rare) stars that do form exhibit top-heavy IMFs. Stars can form only in special locations (e.g. magnetic field switches) in the outer QAD. Protostars accrete their natal cores rapidly but then dynamically decouple from the gas and wander, ceasing accretion on timescales ~100 yr. Their jets control initial core accretion, but the ejecta are swept up into the larger-scale QAD flow without much dynamical effect. The strong tidal environment strongly suppresses common-core multiplicity. The IMF shape depends sensitively on un-resolved dynamics of protostellar disks (PSDs), as the global dynamical times can become incredibly short ($\ll$ yr) and tidal fields are incredibly strong, so whether PSDs can efficiently transport angular momentum or fragment catastrophically at $\lesssim 10$ au scales requires novel PSD simulations to properly address. Most analytic IMF models and analogies with planet formation in PSDs fail qualitatively to explain the simulation IMFs, though we discuss a couple of viable models.

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arXiv2024

Discovery of a Hypervelocity L Subdwarf at the Star/Brown Dwarf Mass Limit

We report the discovery of a high velocity, very low-mass star or brown dwarf whose kinematics suggest it is unbound to the Milky Way. CWISE J124909.08+362116.0 was identified by citizen scientists in the Backyard Worlds: Planet 9 program as a high proper motion ($μ$ $=$ 0''9/yr) faint red source. Moderate resolution spectroscopy with Keck/NIRES reveals it to be a metal-poor early L subdwarf with a large radial velocity ($-$103$\pm$10 km/s), and its estimated distance of 125$\pm$8 pc yields a speed of 456$\pm$27 km/s in the Galactic rest frame, near the local escape velocity for the Milky Way. We explore several potential scenarios for the origin of this source, including ejection from the Galactic center $\gtrsim$3 Gyr in the past, survival as the mass donor companion to an exploded white dwarf. acceleration through a three-body interaction with a black hole binary in a globular cluster, and accretion from a Milky Way satellite system. CWISE J1249+3621 is the first hypervelocity very low mass star or brown dwarf to be found, and the nearest of all such systems. It may represent a broader population of very high velocity, low-mass objects that have undergone extreme accelerations.

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arXiv2024

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

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 masses ($\gtrsim30M_\odot$), 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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