A cosmological framework for stellar collisions at high redshift in proto-globular clusters, nuclear star clusters, and Little Red Dots

Claire E. Williams, Smadar Naoz, Sanaea C. Rose, Blakesley Burkhart, Naoki Yoshida, Avi Chen, Kyle Kremer, William Lake, Federico Marinacci, Shyam H. Menon, Mark Vogelsberger

First listed 2026-03-27 | Last updated 2026-03-27

Abstract

Observations and cosmological simulations indicate that the early Universe hosted numerous compact, high-density stellar systems, where close encounters and physical collisions between stars were likely common. We develop a bottom-up framework for stellar dynamics in such environments, spanning systems with and without intermediate- and supermassive black holes, and covering regimes where stellar collisions may or may not dominate the evolution. This radially-resolved analytic model connects dense star clusters in their cosmological context to observable outcomes mediated by stellar collisions. Initial conditions and environmental properties are drawn from high-resolution cosmological simulations, enabling exploration across a broad region of parameter space. The analytic predictions are validated against Monte Carlo simulations, demonstrating good agreement across key regimes. We find that stellar collisions are ubiquitous in many high-redshift environments, with runaway sequences naturally leading to the formation of very massive stars at early times. Finally, we show that high rates of destructive collisions can rapidly build up extremely dense gaseous environments around massive black holes, potentially providing an analogue to the observed population of Little Red Dots.

Short digest

Bottom-up, radially resolved analytic framework for stellar dynamics in compact high-redshift systems (proto-globulars, nuclear clusters, and LRD-like nuclei), initialized from high-resolution cosmological simulations and validated against Monte Carlo predictions. Across wide cluster/BH parameter space, main-sequence stellar collisions are ubiquitous and naturally drive runaway growth into very massive stars. In systems with central black holes, inner radii are collision-destructive, rapidly converting stellar mass into dense gas that can feed the BH and yield LRD-like compact environments. The framework ties cluster structure to observable outcomes, outlining a cosmological route from dense clusters to early massive seeds.

Key figures to inspect

• Figure 1 — Use the schematic to locate where constructive vs destructive collisions operate for clusters with and without a central BH, and to trace how merger products migrate inward to assemble a VMS versus how inner destructive zones around BHs build a dense gas reservoir.
• Figure 2 — Read the flowchart to understand the model’s decision branches and which outcomes are realized for the explored parameters (grey branches absent; yellow branches only with inner destructive collisions), clarifying when systems evolve toward VMS growth versus BH-fed gas buildup.
• Figure 3 — Examine the radial collision counts and timescale crossings (t_coll vs t_MS, t_merge, t_relax) with/without a BH to identify the radii enabling runaway sequences and how a central BH steepens inner destructive-collision rates within ~pc scales.
• Figure 4 — Compare predicted maximum VMS mass versus half-mass density to prior fits/simulations, then inspect the implied BH–stellar mass relation (assuming full VMS→BH conversion) against dwarf AGN, JWST high‑z AGN, the Milky Way, and IMBH candidates to gauge seeding efficiency in dense clusters.