ALMA finds globular cluster in the making

ALMA finds globular cluster in the making

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Globular clusters are the dinosaurs of the universe. A globular cluster is a huge, ball-shaped collection of many hundreds of thousands of stars – something like a cosmic disco ball. Like dinosaurs, most globular clusters are very large. And also like dinosaurs, they were 'born' long, long ago, when the universe was in its infancy.

Surprisingly, astronomers working with ALMA have found a globular-cluster-in-the-making, in a pair of colliding galaxies some 50 million light-years away. It's as if you found a dinosaur egg in your backyard, ready to hatch.


A globular cluster can only form from an incredibly dense and massive gas cloud, large enough to hold the material for maybe millions of stars. When the universe was young, such huge gas clouds were pretty common. That's why all globular clusters were born billions of years ago.

ALMA has now found a dense and massive gas cloud in the Antenna galaxies – two colliding galaxies that are in the process of merging into one single galaxy. Because of tidal forces and shock waves, the gas in the galaxies is compressed into huge blobs from which new stars will form within the next million years or so.

One of the blobs, nicknamed the Firecracker, appears large enough to produce a new globular cluster in the future. The Firecracker is less than 150 light-years across. Yet, it contains as much matter as fifty million stars like our own sun. Moreover, the gas pressure in the cloud is 10,000 times higher than in 'normal' interstellar gas clouds.

ALMA observed the millimeter radiation from carbon monoxide gas in the Firecracker. By studying this radiation, astronomers could deduce the total mass and the pressure of the cloud. They also found that no new stars have yet been born in the cloud. It's really a 'cosmic egg', ready to hatch.

Observing the formation of a globular cluster is a bit like witnessing the birth of a Tyrannosaurus rex – something you would only expect to happen in the very distant past. Seeing it happen right now may learn astronomers a lot about things that occurred long, long ago.

The Antenna galaxies (also known as NGC 1238 and NGC 1239) are two galaxies that have collided with each other some 600 million years ago. They are located at a distance of about 50 million light-years, in the constellation Corvus, the Raven. As a result of their mutual gravity, long 'tails' of gas and new-born stars have been flung out into space. In the future, the two galaxies will merge into one large galaxy.
The ALMA observations of the 'Firecracker' – the dense and massive gas cloud in the Antenna galaxies from which a new globular cluster may form – were carried out by a team of astronomers led by Kelsey Johnson of the University of Virginia in Charlottesville. Kelsey worked together with her Virginia colleagues Rémy Indebetouw and Aaron Evans, and with astronomers from the National Radio Astronomy Observatory and from the Space Telescope Science Institute. The new results will be published in The Astrophysical Journal.