Protocluster of Super Stars About to ‘Hatch’

Massive star clusters can pop into existence in a matter of a few million years, a very short period of time on astronomical time scales.  They consist of hundreds or thousands of massive, bright, hot stars that will live relatively short lives of a few hundred millions of years.  Using the Atacama Large Millimeter/Submillimeter Array (ALMA), astronomers have discovered a vanishingly rare molecular cloud of highly dense gas, containing no stars.  It is poised to become a massive star cluster, and we found it in its infancy.

This is an ALMA image of dense cores of molecular gas in the Antennae galaxies. The round yellow object near the center may be the first prenatal example of a globular cluster ever identified. It is surrounded by a giant molecular cloud. Credit: K. Johnson, U.Va.; ALMA (NRAO/ESO/NAOJ)

“We may be witnessing one of the most ancient and extreme modes of star formation in the universe,” said Kelsey Johnson, an astronomer at the University of Virginia in Charlottesville and lead author on a paper accepted for publication in theAstrophysical Journal. “This remarkable object looks like it was plucked straight out of the very early universe. To discover something that has all the characteristics of a globular cluster, yet has not begun making stars, is like finding a dinosaur egg that’s about to hatch.”

The Antennae galaxies, shown in visible light in a Hubble image (upper image), were studied with ALMA, revealing extensive clouds of molecular gas (center right image). One cloud (bottom image) is incredibly dense and massive, yet apparently star free, suggesting it is the first example of a prenatal globular cluster ever identified. Credit: NASA/ESA Hubble, B. Whitmore (STScI); K. Johnson, U.Va.; ALMA (NRAO/ESO/NAOJ); B. Saxton (NRAO/AUI/NSF)

The best place to find these formations is in the dense, gas-rich environments of colliding galaxies.  As two galaxies collide the stars that make up their visible material pass right by each other and interact through gravity, but the thick clouds of gas and dust collide head on, creating massive bursts of star formation, aptly named starbursts.  Incidentally this is where the future cluster was discovered, in the colliding regions of two galaxies called the antennae.

ALMA data indicates that the molecular cloud is under extreme pressure, 10,000 times greater than typical pressures found in the interstellar medium, supporting theories that high pressures are required to form dense clusters.

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