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Fossil star clusters reveal their age

A new survey of globular clusters show many of the star groups are almost as old as the universe itself.
NGC 6535
Globular clusters like NGC 6535, found 22,000 light-years away in the constellation Serpens, are groups of ancient stars. A new study indicates these tightly-bound populations are almost as old as the universe itself.
ESA/Hubble & NASA, Acknowledgement: Gilles Chapdelaine
Using a new age-dating method and the W. M. Keck Observatory on Maunakea, an international team of astronomers have determined that ancient star clusters formed in two distinct epochs — the first 12.5 billion years ago and the second 11.5 billion years ago. These results are being published in Monthly Notices of the Royal Astronomical Society.

Although the clusters are almost as old the universe itself, these age measurements show the star clusters — called globular clusters — are actually slightly younger than previously thought.

“We now think that globular clusters formed alongside galaxies rather than significantly before them,” said research team leader Duncan Forbes of the Swinburne University of Technology.

The new estimates of the star cluster average ages were made possible using data obtained from the SAGES Legacy Unifying Globulars and GalaxieS (SLUGGS) survey, which was carried out on Keck Observatory’s 10-meter Keck II telescope. Observations were carried out over years using the powerful DEIMOS multi-object spectrograph fitted on Keck II, which is capable of obtaining spectra of one hundred globular clusters in a single exposure.

DEIMOS breaks the visible wavelengths of objects into spectra, which the team used to reverse-engineer the ages of the globular clusters by comparing the chemical composition of the globular clusters with the chemical composition of the universe as it changes with time.

“The universe is now well known to be 13.7 billion years old,” research team member and professor Jean Brodie said. “We determined globular clusters form on average some 1.2 and 2.2 billion years after the Big Bang.”

“Our age measurements indicate that globular clusters managed to avoid the period, called cosmic re-ionization, in which the universe was bathed in ultraviolet radiation which could have destroyed them,” said fellow team member and professor, Aaron Romanowsky.

“Now that we have estimated when globular clusters form, we next need to tackle the questions of where and how they formed,” Forbes said.

The SLUGGS survey is comprised of an international team of astronomers who aim to understand the formation and evolution of galaxies and their globular cluster systems.

Globular clusters are tightly bound clusters of around a million stars. Most large galaxies, including the Milky Way, host a system of globular clusters. Although the universe itself, and galaxies within it, has evolved over cosmic time, globular clusters are very robust, and many have survived intact for over 10 billion years.

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