From nuclear star clusters to Little Red Dots: black hole growth, mergers, and tidal disruptions

TL;DR

This paper models the growth and activity of black holes in early galactic nuclei, predicting tidal disruption events and black hole captures in Little Red Dots observed by JWST, and discusses their low X-ray luminosity.

Contribution

It introduces a simple nuclear star cluster model that self-consistently simulates black hole growth, tidal disruptions, and gas accretion in early universe conditions.

Findings

Predicted tens of tidal disruption events at redshifts 4-6.

Estimated black hole capture rates are lower than tidal disruption events.

Discussed the implications of low X-ray luminosity of Little Red Dots.

Abstract

Little Red Dots, discovered by the James Webb Space Telescope, are hypothesized to be active galactic nuclei containing a supermassive black hole, possibly surrounded by a dense stellar cluster, large amounts of gas, and likely by a population of stellar-mass black holes. We develop a simple nuclear star cluster model to evolve the rapid mass growth of black hole seeds into the supermassive regime. The combined processes of tidal disruption events, black hole captures, and gas accretion are accounted for self-consistently in our model. Given the observed number density of Little Red Dots, and under reasonable assumptions, we predict at least a few tens of tidal disruption events and at least a few black hole captures at $z=4$-$6$, with a tidal disruption event rate an order of magnitude larger than the black hole capture rate. We also estimate the uncertainties in these estimates. Finally, we comment on the low x-ray luminosity of Little Red Dots.