On the Fate of Little Red Dots

TL;DR

The paper tackles the fate of Little Red Dots (LRDs) observed by JWST under a stellar‑only interpretation. It develops a timescale framework using $t_{relax}$, $t_{coll}$, and $t_{age}$, and corroborates with recent dense‑stellar‑system simulations that enable runaway collisions and MBH seed formation. The authors find that when $t_{age} \sim t_{coll} < t_{relax}$, core runaway collisions efficiently form a Very Massive Star that can collapse into a massive black hole, with MBH formation efficiencies described by a sigmoidal relation involving $M/M_{crit}$. Predicted MBH masses span from roughly $5\times10^6 M_\odot$ up to $10^{10} M_\odot$, placing LRDs as prime sites for in‑situ MBH seed formation, with significant implications for early SMBH growth and prospects of LISA detections from VMS‑collapse events. The framework unifies different interpretations of LRDs by centering MBH seed formation in their dynamical evolution and highlights observational signatures across X‑ray, gravitational wave, and high‑redshift quasar contexts.

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

Figures (2)

• Figure 1: Total masses M and sizes R in color points for the following systems: resolved MBHs (black), unresolved MBHs (white), NSCs (orange) and LRDs (red). The solid black curve follows the condition $\rm t_{coll} (M, R) = t_{H0}$ for a system of a given mass M and radius R (at the current age of the universe $\rm t_{H0} = 1.4 \times 10^{10}$ yrs) and the dashed one, the condition $\rm t_{relax} (M, R) = t_{H0}$. The color diagram denotes the black hole efficiency expected from $\epsilon_{BH} = \left(1 + \exp\left[-4.63 \left( \log\left(M/M_{crit}\right) - 4 \right) \right]\right)^{-0.1}$ (M.C. Vergara et al 2023, 2024, 2025b), with $\rm M_{crit}$ coming from the condition $\rm t_{coll} \,(M_{crit}) = t_{H0}$. Those efficiencies match the expectations for the MBH formation scenario outlined in Escala (2021), where MBHs are formed from (partially/totally) failed stellar systems, by approaching towards 1 for resolved MBHs (black points) and being NSCs mainly in the range between 0.05 and 0.15 (coexistence of a MBH and a NSC; Escala 2021). The region called 'Forbidden Stellar Zone' fulfill the instability defined by $\rm t_{coll} < t_{H0} \, (< t_{relax})$, where no stellar system is expected to survive, being most LRDs at the boundary of such instability (within one order of magnitude if we consider their upper limits in total effective radius).
• Figure 2: (a) Total masses M and radius R in red points for LRDs, the solid black curve following the condition $\rm t_{coll} (M, R) = t_{H0}$ for the age of the universe at z$\sim$8 ($t_{H0} = 0.6 \times 10^{9}$ yrs) and the dashed one, the condition $\rm t_{relax} (M, R) = t_{H0}$. The color diagram denotes the predicted black hole mass $\rm M_{BH}$ expected from $\rm M \times \left(1 + \exp\left[-4.63 \left( \log\left(M/M_{crit}\right) - 4 \right) \right]\right)^{-0.1}$ (Vergara et al 2023, 2024, 2025b), predicting $\rm M_{BH}$ an approximate range between $\rm 5\times 10^6M_{\odot}$ and $10^{10}M_{\odot}$. (b) The positions of LRDs for the measured upper limit radius in red, in green LRDs positions assuming that their real effective radii corresponds to the 10% of the upper limits and in cyan, assuming that corresponds to the 1% of the upper limits in radius. The solid and dashed black curves denotes the same condition as in (a), but at the cosmic age of appearance ($t_{H0} = 0.6\times 10^9$yr) and disappearance ($t_{H0} = 1.5\times 10^9$yr) of LRDs. In the case of the green points, a fraction enters to the 'Forbidden Stellar Zone' ($t_{coll} < t_{H0} < t_{relax}$), while the cyan points it is more extreme, with most LRDs are in such situation. (c) Same as (b), now assuming that it is the core radius of the LRDs the one equals to the 10% (green) or 1% (cyan) of their measured effective radii, with most of the cores entering to the 'Forbidden Stellar Zone' for the case where LRDs core radius equals to the 1% of their measured radii.