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Peer-reviewedCC BY 4.0The Astrophysical Journal

From Feedback-free Star Clusters to Little Red Dots via Compaction

  • Avishai Dekel1
  • Dhruba Dutta Chowdhury1
  • Sharon Lapiner1
  • Zhiyuan Yao1
  • Shmuel Gilbaum1
  • Daniel Ceverino2
  • Joel Primack3
  • Rachel Somerville4
  • Romain Teyssier5
5 affiliations
  1. 1The Hebrew University
  2. 2Universidad Autonoma de Madrid
  3. 3University of California
  4. 4Flatiron Institute
  5. 5Princeton University
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Abstract

We address the origin of the little red dots (LRDs) seen by JWST at cosmic morning ( z = 4–8) as compact stellar systems with overmassive black holes (BHs). We propose that LRDs form naturally after feedback-free starbursts (FFB) in thousands of star clusters and following wet compaction. Analytically, we show how the clusters enable efficient dry migration of stars and BHs to galaxy centers by two-body segregation and dynamical friction against the disk. The clusters merge to form compact central stellar systems as observed. Mutual tidal stripping does not qualitatively affect the analysis. The young, rotating clusters are natural sites for the formation of BH seeds via rapid core collapse. The migrating clusters carry the BH seeds, which merge into central supermassive black holes (SMBHs). Compactions are required to deepen the potential wells such that the SMBHs are retained after postmerger gravitational-wave recoils, locked to the galaxy centers. Using cosmological simulations at different epochs, with different codes and physical recipes, we evaluate the additional growth of LRD-matching compact central stellar systems by global compaction events. Adding to the dry growth by cluster mergers, the compactions can increase the escape velocities to retain the SMBHs. The LRDs appear at z ∼ 8, after the formation of FFB clusters, and disappear after z ∼ 4 when the stellar mass is above 10 ⁹ M ⊙ by growing postcompaction blue disks around the nuclear LRDs. The LRD abundance is expected to be ∼10 ⁻⁵ –10 ⁻⁴ Mpc ⁻³ , increasing from z ∼ 4 to z ∼ 8.

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Cite

BibTeXRIS
BibTeX
@article{dekel2026from,
  title = {{From Feedback-free Star Clusters to Little Red Dots via Compaction}},
  author = {Avishai Dekel and Dhruba Dutta Chowdhury and Sharon Lapiner and Zhiyuan Yao and Shmuel Gilbaum and Daniel Ceverino and Joel Primack and Rachel Somerville and Romain Teyssier},
  journal = {The Astrophysical Journal},
  year = {2026},
  month = oct,
  doi = {10.3847/1538-4357/aea145},
  url = {https://iopscience.iop.org/article/10.3847/1538-4357/aea145},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - From Feedback-free Star Clusters to Little Red Dots via Compaction
AU  - Avishai Dekel
AU  - Dhruba Dutta Chowdhury
AU  - Sharon Lapiner
AU  - Zhiyuan Yao
AU  - Shmuel Gilbaum
AU  - Daniel Ceverino
AU  - Joel Primack
AU  - Rachel Somerville
AU  - Romain Teyssier
PY  - 2026
DA  - 2026/10/16
JO  - The Astrophysical Journal
DO  - 10.3847/1538-4357/aea145
UR  - https://iopscience.iop.org/article/10.3847/1538-4357/aea145
AB  - We address the origin of the little red dots (LRDs) seen by JWST at cosmic morning ( z = 4–8) as compact stellar systems with overmassive black holes (BHs). We propose that LRDs form naturally after feedback-free starbursts (FFB) in thousands of star clusters and following wet compaction. Analytically, we show how the clusters enable efficient dry migration of stars and BHs to galaxy centers by two-body segregation and dynamical friction against the disk. The clusters merge to form compact central stellar systems as observed. Mutual tidal stripping does not qualitatively affect the analysis. The young, rotating clusters are natural sites for the formation of BH seeds via rapid core collapse. The migrating clusters carry the BH seeds, which merge into central supermassive black holes (SMBHs). Compactions are required to deepen the potential wells such that the SMBHs are retained after postmerger gravitational-wave recoils, locked to the galaxy centers. Using cosmological simulations at different epochs, with different codes and physical recipes, we evaluate the additional growth of LRD-matching compact central stellar systems by global compaction events. Adding to the dry growth by cluster mergers, the compactions can increase the escape velocities to retain the SMBHs. The LRDs appear at z ∼ 8, after the formation of FFB clusters, and disappear after z ∼ 4 when the stellar mass is above 10 ⁹ M ⊙ by growing postcompaction blue disks around the nuclear LRDs. The LRD abundance is expected to be ∼10 ⁻⁵ –10 ⁻⁴ Mpc ⁻³ , increasing from z ∼ 4 to z ∼ 8.
SN  - 0004-637X
SN  - 1538-4357
ER  -