The emergence of globular clusters and globular-cluster-like dwarfs - Nature

"GCs were first discovered by Abraham Ihle in 1665, yet there is still no consensus on how they form1. They are among the densest stellar systems known, with stellar masses M* ≈ 10 5-6  M⊙ and half-light radii R1/2 ≈ 1-10 pc (ref. 1), making them an important source of gravitational waves5 and possible sites for the formation of supermassive black holes seeds6,7,8. Most are assumed to have no dark matter, suggesting that they form along a distinct pathway to dark-matter-rich dwarf galaxies9,10,11."

"Current theories for GC formation fall into three main categories. The first suggests that they form in the same way as all star clusters but are simply the high-mass tail of the distribution12. The second suggests that they require special conditions to form, such as galaxy mergers13,14, high-density converging gas flows15,16, low-metallicity gas17,18 or disk instabilities19. The third suggests that they form inside their own dark-matter halos20,21,22,23,24,25."

Globular clusters are extremely dense stellar systems with masses of order 10^5–10^6 M⊙ and half-light radii of 1–10 pc, and they can be sources of gravitational waves and potential sites for early supermassive-black-hole seed formation. Most globular clusters appear to lack dark matter, distinguishing them from dwarf galaxies that are dark-matter dominated, have extended star formation, and show large metallicity spreads. Proposed formation channels include being the high-mass tail of normal cluster formation, forming under special conditions (mergers, dense gas flows, low metallicity, disk instabilities), or forming inside their own dark-matter halos. The coeval formation of globulars and low-mass dwarfs and objects occupying overlapping size–luminosity space remain open puzzles.

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