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Little Red Dots Are Nurseries of Massive Black Holes

Fabio Pacucci, Lars Hernquist, Michiko Fujii

TL;DR

This work investigates whether extreme stellar densities in the cores of Little Red Dots (LRDs) at $z\sim5$ can drive runaway stellar collisions to form massive black hole seeds. Using a Fokker–Planck approach, an analytical VMS-growth model, and direct $N$-body simulations, the authors show that rapid mass segregation and central collisional growth produce a very massive star (VMS) of $M_{\rm VMS} \sim 9\times10^{3}-5\times10^{4}\,M_\odot$ within $\lesssim 1$ Myr, followed by Kelvin–Helmholtz contraction and general-relativistic collapse to a black hole of $M_\bullet \sim 10^{4}\,M_\odot$. This path provides a robust channel for heavy black hole seed formation in the early universe, potentially yielding higher seed number densities than direct-collapse models and sustaining tidal disruption–driven activity in dense cores. The results imply LRDs could seed the SMBHs observed at later times, while highlighting uncertainties tied to stellar masses, densities, and observational biases. Overall, the paper positions dense stellar dynamics in LRDs as a competitive and widespread mechanism for early massive black hole formation.

Abstract

The James Webb Space Telescope (JWST) has revealed a previously unknown population of compact, red galaxies at $z \sim 5$, known as "Little Red Dots" (LRDs). With effective radii of $\sim 100$ pc and stellar masses of $10^9-10^{11} \, M_\odot$, a purely stellar interpretation implies extreme central densities, $ρ_\star\sim10^4-10^5 \, M_\odot \, \mathrm{pc}^{-3}$ and in some cases up to $\sim 10^9 \, M_\odot \, \mathrm{pc}^{-3}$, far exceeding those of globular clusters. At such densities, the dynamical friction time for $10 \, M_\odot$ stars in the central $0.1$ pc is $< 0.1$ Myr, driving rapid mass segregation. We investigate the dynamical consequences of such an environment using: (i) a Fokker-Planck analysis of long-term core evolution, (ii) an analytical model for the collisional growth of a very massive star (VMS), and (iii) direct $N$-body simulations. All approaches show that runaway collisions produce a VMS with mass $9\times10^3 < M_{\rm VMS} \, [M_\odot] < 5\times10^4$ within $<1$ Myr. Once the supply of massive stars is depleted, the VMS contracts on a $\sim 8000$ yr Kelvin-Helmholtz timescale and undergoes a general relativistic collapse, leaving a massive black hole of mass $M_\bullet \sim 10^4 \, M_\odot$. We conclude that LRDs are natural nurseries for the formation of heavy black hole seeds via stellar-dynamical processes. This pathway produces seed number densities that far exceed those expected from direct collapse models, and, owing to the dense residual stellar core, can sustain high rates of tidal disruption events.

Little Red Dots Are Nurseries of Massive Black Holes

TL;DR

This work investigates whether extreme stellar densities in the cores of Little Red Dots (LRDs) at can drive runaway stellar collisions to form massive black hole seeds. Using a Fokker–Planck approach, an analytical VMS-growth model, and direct -body simulations, the authors show that rapid mass segregation and central collisional growth produce a very massive star (VMS) of within Myr, followed by Kelvin–Helmholtz contraction and general-relativistic collapse to a black hole of . This path provides a robust channel for heavy black hole seed formation in the early universe, potentially yielding higher seed number densities than direct-collapse models and sustaining tidal disruption–driven activity in dense cores. The results imply LRDs could seed the SMBHs observed at later times, while highlighting uncertainties tied to stellar masses, densities, and observational biases. Overall, the paper positions dense stellar dynamics in LRDs as a competitive and widespread mechanism for early massive black hole formation.

Abstract

The James Webb Space Telescope (JWST) has revealed a previously unknown population of compact, red galaxies at , known as "Little Red Dots" (LRDs). With effective radii of pc and stellar masses of , a purely stellar interpretation implies extreme central densities, and in some cases up to , far exceeding those of globular clusters. At such densities, the dynamical friction time for stars in the central pc is Myr, driving rapid mass segregation. We investigate the dynamical consequences of such an environment using: (i) a Fokker-Planck analysis of long-term core evolution, (ii) an analytical model for the collisional growth of a very massive star (VMS), and (iii) direct -body simulations. All approaches show that runaway collisions produce a VMS with mass within Myr. Once the supply of massive stars is depleted, the VMS contracts on a yr Kelvin-Helmholtz timescale and undergoes a general relativistic collapse, leaving a massive black hole of mass . We conclude that LRDs are natural nurseries for the formation of heavy black hole seeds via stellar-dynamical processes. This pathway produces seed number densities that far exceed those expected from direct collapse models, and, owing to the dense residual stellar core, can sustain high rates of tidal disruption events.

Paper Structure

This paper contains 12 sections, 18 equations, 4 figures, 1 table.

Table of Contents

  1. Introduction
  2. Methods
  3. Fokker-Planck Analysis
  4. Analytical Calculation
  5. N-body Simulations
  6. Results
  7. Fokker-Planck Analysis: Results
  8. Analytical Calculation: Results
  9. $N$-body Simulations: Results
  10. Bracketing the Final VMS Mass
  11. Massive Black Hole Formation
  12. Discussion and Conclusions

Figures (4)

  • Figure 1: Left: Radial density profiles at initial, final, and two intermediate collapse times, labeled by fraction of the global relaxation time $t_{\rm rel}$. Right: Corresponding enclosed mass profiles. The central parsec gains four orders of magnitude in mass, while the outer layer remains static.
  • Figure 2: Dynamical friction time and velocity dispersion before (dark) and after (light) core contraction. The dynamical friction time $t_{\rm fric}$ is calculated for a massive star of $10 \,{\rm M_\odot}$; in the central $0.1$ pc of the core, it falls to $< 0.1$ Myr, enabling rapid mass segregation. The red curves (right axis) show the one‐dimensional velocity dispersion $\sigma(r)$.
  • Figure 3: Time evolution of the mass of the VMS (left axis) and the mass accretion and loss rates (right axis) onto it, calculated in our analytical model. The effective rate of growth of the VMS is given by the (shaded) difference between mass accretion and mass loss rates. The final mass, $M_{\rm VMS} = 5\times 10^4 \,{\rm M_\odot}$, is reached in a time comparable to the dynamical friction timescale in the central $0.1$ pc of the core.
  • Figure 4: Time evolution of the VMS mass in the direct $N$-body simulation. The system contains $10^6$ stars drawn from a Kroupa IMF and is initialized with a central stellar density of $10^7 \, \,{\rm M_\odot} \, \mathrm{pc}^{-3}$. The early phase shows discrete merger steps, followed by a runaway growth episode after $\sim 0.5$ Myr that drives the VMS to $9000 \,{\rm M_\odot}$ within $\sim 1$ Myr. The shaded region indicates the cumulative mass growth. The instantaneous mass accretion rate is also displayed in red, on the right vertical axis.