From Feedback-Free Star Clusters to Little Red Dots via Compaction

Avishai Dekel, Dhruba Dutta Chowdhury, Sharon Lapiner, Zhiyuan Yao, Shmuel Gilbaum, Daniel Ceverino, Joel Primack, Rachel Somerville, Romain Teyssier

First listed 2025-11-10 | Last updated 2025-11-10

Abstract

We address the origin of the Little Red Dots (LRDs) seen by JWST at cosmic morning ($z \!=\! 4 \!-\! 8$) as compact stellar systems with over-massive black holes (BHs). We propose that LRDs form naturally after feedback-free starbursts (FFB) in thousands of star clusters and following wet compaction. Analytically, we show how the clusters enable efficient dry migration of stars and BHs to the galaxy center by two-body segregation and dynamical friction against the disk. The clusters merge to form compact central clusters as observed. Mutual tidal stripping does not qualitatively affect the analysis. The young, rotating clusters are natural sites for the formation of BH seeds via rapid core collapse. The migrating clusters carry the BH seeds, which merge into central super-massive BHs (SMBHs). Compactions are required to deepen the potential wells such that the SMBHs are retained after post-merger gravitational-wave recoils, locked to the galaxy centers. Using cosmological simulations at different epochs, with different codes and physical recipes, we evaluate the additional growth of LRD-matching compact central stellar systems by global compaction events. Adding to the dry growth by cluster mergers, the compactions can increase the escape velocities to retain the SMBHs. The LRDs appear at $z \!\sim\! 8$, after the formation of FFB clusters, and disappear after $z \!\sim\! 4$ when the stellar mass is above $10^9 M_\odot$ by growing post-compaction blue disks around the nuclear LRDs. The LRD abundance is expected to be $\sim\! 10^{-5} \!-\! 10^{-4}\,{\rm Mpc}^{-3}$, increasing from $z \!\sim\! 4$ to $z\!\sim\! 8$.

Short digest

Proposes a concrete formation path for Little Red Dots at z≈4–8: feedback-free starbursts create thousands of young, rotating clusters whose stars and seed black holes dry-migrate to the galactic center via two-body segregation and disk dynamical friction, building a compact nucleus. Wet compaction events then deepen the potential well, boosting escape velocities so coalescing black holes are retained against gravitational-wave recoils and locked to the center, while also growing the dense stellar core. Guided by analytics plus cosmological simulations, the model predicts LRDs emerge by z∼8 and fade by z∼4 as blue disks regrow, with number densities ≈10⁻⁵–10⁻⁴ Mpc⁻³ that increase toward higher redshift. This links over-massive BHs and extreme stellar densities without requiring strong X-ray/radio signatures.

Key figures to inspect

• Figure 1: Use the mass–timescale curves to see which mechanism (two-body segregation vs disk/halo dynamical friction) dominates inward transport for FFB-like clusters, and whether migration completes within a few hundred Myr at z∼6.
• Figure 2: Track how repeated cluster mergers change R_half, 3D/surface densities, and the central escape velocity; compare equal-mass vs minor-merger sequences to judge when v_esc approaches the recoil-retention threshold once halo+gas contributions are included.
• Figure 3: Inspect tidal-encounter tracks to verify clusters largely survive the migration time while developing a smoothed component only at higher encounter speeds; note where impulse and tidal-limit assumptions remain valid.
• Figure 4: Read off the escape-velocity requirement for sustained SMBH growth across seed-mass slopes and disk “temperature”; compare the inferred threshold to pre- and post-compaction v_esc expected for FFB galaxies to see why compaction is necessary.