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 (50–200 M⊙) sink to the core and merge over many Gyr.

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 ~47,000 (Noyola 2008), with the JWST accretion non-detection suggesting >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 allowed window spans roughly 8,200 to 70,000 M⊙, with the most likely values clustering around 20,000–50,000 M⊙.

The formation scenarios from Step 2 produce different expected present-day masses. The compact metal-poor scenario predicts ~100,000 M⊙ — already in tension with some upper limits. The ω Cen-as-observed scenario predicts ~50,000 M⊙, comfortably inside the current window. Load the Constraint Stacker and overlay the formation-channel prediction to see where each scenario sits.