On the Fate of Little Red Dots

Andres Escala, Lucas Zimmermann, Sebastian Valdebenito, Marcelo C. Vergara, Dominik R. G. Schleicher, Matias Liempi

First listed 2025-09-24 | Last updated 2025-11-30

Abstract

We study the stability and possible fates of Little Red Dots, under the stellar-only interpretation of their observational features. This is performed by a combination of analyzing the relevant timescales in their stellar dynamics and also, the application of recent numerical results on the evolution of the densest stellar systems. We find that these objects typically have tage ~ tcoll < trelax, therefore, in an unexplored regime never observed before for a stellar system and potentially, highly unstable to runaway collisions. We study different scenarios for the evolution of Little Red Dots and conclude that in a fair fraction of those systems, the formation of a massive black hole by runaway collisions seems unavoidable, in all the possibilities studied within the stellar-only interpretation. This evolutionary path would naturally explain many of the problematic characteristics of Little Red Dots, including that these objects are probably transient in the history of the Universe, that most of them would not emit X-rays since they would not yet have become massive black holes, and once they do, they would constitute a significant portion of the mass of the Little Red Dots. We conclude that Little Red Dots are the most favourable known places to find a recently formed massive black hole seed, or in the process of formation, most probably formed directly in the supermassive range

Short digest

Analyzes Little Red Dots in a stellar-only framework by mapping collision, relaxation, and age timescales and folding in recent N-body results for extreme stellar systems. Finds typical LRD cores sit in the tage ~ tcoll < trelax regime, triggering core runaway collisions that form a massive black hole; in more extreme cases with tcoll < trelax the whole system enters a “Forbidden Stellar Zone” and collapses. This pathway yields high BH formation efficiencies (~10–50% of the stellar mass), naturally explaining LRD transience, frequent X-ray non-detections prior to seed formation, and over-massive remnants once formed. Concludes LRDs are prime sites to catch nascent, potentially already supermassive, BH seeds at z≈4–8.

Key figures to inspect

• Timescale map in the M–R plane with contours of tcoll and trelax and shading of the ‘Forbidden Stellar Zone’; verify that the LRD parameter box lies where tage ~ tcoll < trelax and inspect boundaries where full collapse is expected.
• MBH formation efficiency versus tcoll/tage (from the cited N-body results); check how efficiencies approach 10–50% and where LRD-like densities/velocities land on the curve.
• Predicted velocity dispersion versus compactness for R≈100 pc cores; compare σ-derived line widths (~1500 km s⁻1) to Balmer broad components to assess a stellar-dispersion origin without an active BH.
• Evolutionary pathways schematic contrasting trelax < tcoll (NSC + central MBH) against tcoll < trelax (global collapse); note expected observables: X-ray quiet pre-seed phase and rapid transition to an over-massive MBH.