Supermassive stars with embedded stellar black hole cores: dense assembling star clusters as faint multiple Little Red Dot systems

Antti Rantala

First listed 2026-04-24 | Last updated 2026-04-24

Abstract

Numerical simulations have established that star clusters with densities comparable to the high redshift ($z>6$-$10$) James Webb Space Telescope (JWST) proto globular clusters may build up extremely massive (EMSs; $m_\mathrm{\star}>1000 M_\odot$) or even supermassive stars (SMSs; $m_\mathrm{\star}>10000 M_\odot$) and potentially intermediate mass black holes (IMBHs) through runaway stellar collisions. Using direct simulations of assembling star clusters including post-Newtonian black hole dynamics and stellar evolution, we demonstrate that in such dense environments ($Σ_\mathrm{h} \gtrsim 10^6 M_\odot$pc$^\mathrm{-2}$) stellar BHs ($m_\bullet \lesssim 60 M_\odot$), driven by rapid mass segregation and relaxation effects within the sphere of influence of the EMSs/SMSs, may strongly interact with the extremely massive stars and become embedded within their gaseous layers. We suggest that this quasi-star (QS) like embedded BH phase is a natural outcome of the runaway formation of EMSs/SMSs in the densest star clusters. The QS phase is orders of magnitude longer in duration than the lifetime of the SMS, enabling an extended growth period by stellar collisions, and allows the formation of embedded gravitational wave sources if the QS captures more than a single stellar BH. The star cluster assembly region sizes ($\sim100$ pc), QS masses ($\gtrsim 10^4 M_\odot$) and their proximity to young, massive blue star forming clumps are consistent with the faint population of multiple little red dots (LRDs) recently discovered by the JWST.

Short digest

Direct N-body plus stellar-evolution simulations of assembling clusters with Σ_h ≳ 10^6 M⊙ pc^-2 show that stellar BHs (m_bullet ≲ 60 M⊙) rapidly segregate into the influence radius of collision-grown EMSs/SMSs and become embedded in their envelopes, forming quasi-star–like objects. This QS phase lasts orders of magnitude longer than an SMS lifetime, enabling continued mass growth via stellar collisions and, when multiple BHs are captured, creating embedded gravitational-wave sources. The predicted assembly scales (~100 pc), QS masses (≳10^4 M⊙), and association with nearby blue star-forming clumps align with the faint, multiple-component Little Red Dot population. The work links runaway collisional cluster evolution to early black-hole seed growth consistent with LRD phenomenology.

Key figures to inspect

• Figure 1: Inspect the five interaction categories A–E and how category E (BH+EMS/SMS) separates in stellar and BH mass—this isolates the embedded-BH regime with extreme star–BH mass ratios.
• Figure 2: Trace the six EMS/SMS growth tracks in HD9Z1 vs ID9Z1 (stronger vs weaker winds), noting the collisional rejuvenation that extends lifetimes and the timing of first BH formation relative to when BH–EMS/SMS encounters occur.
• Figure 3: Examine BH orbits within the SMS sphere of influence at the quoted Myr snapshot; the near-Keplerian trajectories and BH counts inside r_infl illustrate why capture/embedding becomes likely.
• Figure 4: Compare scalar resonant and two-body relaxation timescales inside r_infl with the short Myr interaction window; this shows dynamics can deliver stellar BHs onto interaction orbits quickly enough to form QSs.