Little Red Dots Are Nurseries of Massive Black Holes

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, ${\rho }_{\star }\sim 10^4-10^5{M}_{\odot }{\mathrm{pc}}^{-3}$ 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 $10M_{\odot }$ 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 $9\times 10^3<M_{\mathrm{VMS}}[M_{\odot }]<5\times 10^4$ 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.

Accepted for publication in The Astrophysical Journal. 12 pages, 4 figures

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