From nuclear star clusters to Little Red Dots: black hole growth, mergers, and tidal disruptions

Konstantinos Kritos, Joseph Silk

First listed 2025-10-24 | Last updated 2025-10-24

Abstract

Little Red Dots, discovered by the James Webb Space Telescope, are hypothesized to be active galactic nuclei containing a supermassive black hole, possibly surrounded by a dense stellar cluster, large amounts of gas, and likely by a population of stellar-mass black holes. We develop a simple nuclear star cluster model to evolve the rapid mass growth of black hole seeds into the supermassive regime. The combined processes of tidal disruption events, black hole captures, and gas accretion are accounted for self-consistently in our model. Given the observed number density of Little Red Dots, and under reasonable assumptions, we predict at least a few tens of tidal disruption events and at least a few black hole captures at $z=4$-$6$, with a tidal disruption event rate an order of magnitude larger than the black hole capture rate. We also estimate the uncertainties in these estimates. Finally, we comment on the low x-ray luminosity of Little Red Dots.

Short digest

The authors build a simple nuclear star cluster framework in which Little Red Dots host massive black hole seeds that grow through self-consistent tidal disruptions, black hole captures/mergers, and gas accretion. Calibrated to the observed LRD number density, the model predicts at z=4–6 at least a few tens of tidal disruption events and at least a few black hole captures, with the TDE rate roughly an order of magnitude higher than the capture rate. Episodic gas inflow accelerates early growth and correlates with enhanced loss‑cone feeding, offering a path to rapid SMBH assembly while remaining compatible with the generally low X‑ray luminosities of LRDs. The work also quantifies uncertainties on these event-rate estimates.

Key figures to inspect

• Figure 1: Inspect the radial density and velocity-dispersion profiles of stars vs stellar‑mass BHs to see where the stellar cusp and BH subcluster form, how the influence radii are set, and how loss‑cone vs evaporation fluxes pick out where TDEs/EMRIs originate.
• Figure 2: Compare runs with and without gas accretion; the 20 gas‑inflow episodes drive step‑like SMBH growth and simultaneous spikes in loss‑cone rates—use this to gauge how inflow modulates TDE and capture activity over time.
• Figure 3: Use the schematic plus time‑evolution panel to trace mass flow channels (stars, BH remnants, and gas) into the central BH; the inset highlights how discrete inflow episodes imprint on the SMBH growth history.
• Figure 4: Check the cumulative counts and time‑resolved rates of TDEs, EMRI captures, and BH binaries; verify the predicted TDE ≫ capture ratio (~10:1) and that 3‑body binary formation stays sub‑dominant (<1 event).