Little Red Dots are Nurseries of Massive Black Holes
Fabio Pacucci, Lars Hernquist, Michiko S. Fujii
Abstract
Abstract The James Webb Space Telescope has revealed a previously unknown population of compact, red galaxies at z ∼ 5, known as “little red dots” (LRDs). With effective radii of ∼100 pc and stellar masses of 10 9 –10 11 M ⊙ , a purely stellar interpretation implies extreme central densities, ρ ⋆ ∼ 10 4 –10 5 M ⊙ pc −3 and in some cases up to ∼10 9 M ⊙ pc −3 , far exceeding those of globular clusters. At such densities, the dynamical friction time for 10 M ⊙ 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 × 10 3 < M VMS [ M ⊙ ] < 5 × 10 4 within <1 Myr. Once the supply of massive stars is depleted, the VMS contracts on a ∼8000 yr Kelvin–Helmholtz timescale and undergoes a general relativistic collapse, leaving a massive black hole of mass M • ∼ 10 4 M ⊙ . 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.