Why ancient star clusters are all the same size

Why do the universe's oldest star clusters tend to be roughly the same size? New simulations suggest it's because galaxy mergers destroyed the smaller ones.

Globular star clusters are ancient, spherical blobs of stars. Most are a few hundred thousand times the mass of our sun. The scarcity of much bigger clusters is no surprise, as they would form more rarely ? but why are there so few small ones?

Using computer modelling, Diederik Kruijssen of the Max Planck Institute for Astrophysics in Garching, Germany, and his colleagues simulated the merger of two small galaxies. This process is thought to have been especially common in the early universe, when small structures were snowballing into larger ones.

The collision compressed gas into dense knots, sparking star formation and creating many new globular clusters. At the same time it seeded destruction, as the gravity of these knots disrupted other clusters ? both existing and new ? passing nearby. Larger clusters survived, bound tightly by their own strong gravity, but the smaller ones were ripped apart.

All large galaxies are thought to have formed in such mergers, so this might be why globular clusters everywhere fall into a narrow size range, says Kruijssen.

Journal reference: arxiv.org/abs/1112.1065; the work will appear in Monthly Notices of the Royal Astronomical Society

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