How Did the IMBH Form?

ω Centauri is not a typical globular cluster. Its multiple stellar populations with distinct metallicities, retrograde orbit around the Galaxy, abnormally high mass (~4×10⁶ M⊙), and flattened rotation signature all point to a single origin: it is the surviving nucleus of a dwarf galaxy that was tidally disrupted by the Milky Way several billion years ago. Most of the dwarf's stars were stripped away; only its dense nucleus — and the IMBH it had already grown — remain.

This origin matters for the IMBH because dwarf-galaxy nuclei naturally concentrate mass in their centres through dynamic friction and nuclear star-cluster formation. The IMBH was likely growing in the dwarf's nucleus long before the dwarf fell into the Milky Way, giving it a head start relative to a newly formed globular cluster of the same final mass.

Two mechanisms can produce an IMBH seed in a dense cluster. Runaway stellar collisions require rapid core collapse (<3 Myr) and low metallicity so that stellar winds do not ablate the colliding stars. Under those conditions a very massive star (VMS) of hundreds to thousands of solar masses forms within ~1 Myr and collapses directly into an IMBH. Repeated black-hole mergers are slower but universally available: stellar-mass BHs (5–50 M⊙, below the ~50–130 M⊙ pair-instability gap where single massive stars cannot leave a black hole remnant) sink to the core and merge over many Gyr, hierarchical mergers eventually building objects that cross into and beyond the gap.

For ω Cen's observed parameters (moderately metal-poor, rₖ = 7 pc), the tool explores the mixed channel — partial runaway contribution plus ongoing BH mergers. González Prieto et al. 2025 N-body simulations yield a seed of ~500–1,500 M⊙ and a final mass of ~50,000 M⊙. This simplified calculator is approximate; its output should be used to explore parameter sensitivity rather than to replicate the published N-body result exactly.

Once the seed exists, three processes feed it: stellar tidal disruption events (TDEs), in which scattered stars pass within the tidal radius and are shredded; compact-object inspiral, in which stellar-mass BHs and neutron stars spiral in via gravitational-wave emission; and gas accretion, which is episodic and tied to the cluster's star-formation history. González Prieto et al. (2025) ran Monte Carlo N-body models showing that TDE-driven growth dominates in ω Cen-like clusters.

The growth history tool lets you see the cumulative accretion rate, the relative contribution of each channel, and the trajectory from seed to the present-day mass window. The current IMBH mass is not yet settled — estimates span 8,200 (Häberle 2024 kinematic lower bound) to ~40,000 (Noyola et al. 2008, (4.0^+0.75_−1.0)×10⁴ M⊙), with the JWST accretion non-detection setting a mass upper limit that is most constraining below ≈20,000 M⊙ (Chen et al. 2025).

Every formation scenario ends at the same empirical question: what does the data actually allow? The Constraint Stacker overlays all published IMBH mass measurements and upper limits — stellar kinematics, HST proper motions, pulsar timing, JWST accretion, N-body models — and computes the joint posterior on M₂. As of mid-2026 the constraints do not agree: Häberle et al. (2024) set a lower bound of ≥ 8,200 M⊙ from HST/oMEGACat kinematics, while Bañares-Hernández et al. (2025) set an upper bound of ≤ 6,000 M⊙ (3σ) from a combined Jeans-plus-pulsar-timing analysis. The two allowed windows do not overlap, and no single value is yet the consensus mass.

The formation scenarios from Step 2 produce broadly similar expected present-day masses: González Prieto et al. (2025) find that seeds anywhere in the 500–5,000 M⊙ range, including the compact metal-poor channel's larger seed, converge on ∼50,000 M⊙ after 12 Gyr of growth — a figure already in tension with the Bañares upper limit and only marginally consistent with the JWST accretion ceiling. Load the Constraint Stacker and overlay the formation-channel prediction to see where each scenario sits.