The Violent Birth of the 13-Billion-Year-Old Globular Star Clusters Orbiting the Milky Way

 

           SALT_PR47TUC_05

 

When the Milky Way first formed, thousands of globular clusters roamed our Galaxy. Today, there are perhaps 200 remaining, with most destroyed over the eons by repeated fateful encounters with each other or the Galactic center. The surviving clsuters are older than any Earth fossil, older than any other structures in our Galaxy.

At 13 billion years of age, these globular clusters are almost as old as the universe itself and were born when the first generations of stars and galaxies formed. The physicist Edward Teller believed that because of their age, globular clusters  would be the natural place to seek out signals from advanced extraterrestrial intelligence.


The image above shows 47 Tucanae –an ancient cluster of several million stars located about 15,000 light years from Earth. The stars in 47 Tucanae are an estimated 10-12 billion years old, placing them among the oldest in our galaxy. Because the stars in clusters like 47 Tucanae are about the same age, they make perfect laboratories for the study of stellar life cycles.

Now a team of astronomers from Germany and the Netherlands have conducted a novel type of computer simulation that looked at how they were born – and they find that these giant clusters of stars are the only survivors of a 13 billion year-old massacre that destroyed many of their smaller siblings. 

Globular star clusters have an unusual characteristic: the typical number of stars they contain appears to be about the same throughout the Universe. This is in contrast to much younger stellar clusters, which can contain almost any number of stars, from fewer than 100 to many thousands. The team of scientists proposes that this difference can be explained by the conditions under which globular clusters formed early on in the evolution of their host galaxies.

The researchers ran simulations of isolated and colliding galaxies, in which they included a model for the formation and destruction of stellar clusters. When galaxies collide, they often generate spectacular bursts of star formation ("starbursts") and a wealth of bright, young stellar clusters of many different sizes. As a result it was always thought that the total number of star clusters increases during starbursts. But the Dutch-German team found the opposite result in their simulations.

While the very brightest and largest clusters were indeed capable of surviving the galaxy collision due to their own gravitational attraction, the numerous smaller clusters were effectively destroyed by the rapidly changing gravitational forces that typically occur during starbursts due to the movement of gas, dust and stars.

The wave of starbursts came to an end after about 2 billion years and the researchers were surprised to see that only clusters with high numbers of stars had survived. These clusters had all the characteristics that should be expected for a young population of globular clusters as they would have looked about 11 billion years ago.

"It is ironic to see that starbursts may produce many young stellar clusters, but at the same time also destroy the majority of them," observed Kruijssen. "This occurs not only in galaxy collisions, but should be expected in any starburst environment. In the early Universe, starbursts were commonplace – it therefore makes perfect sense that all globular clusters have approximately the same large number of stars. Their smaller brothers and sisters that didn't contain as many stars were doomed to be destroyed."

According to the simulations, most of the star clusters were destroyed shortly after their formation, when the galactic environment was still very hostile to the young clusters. After this episode ended, the surviving globular clusters have lived quietly until the present day.

"In the nearby Universe, there are several examples of galaxies that have recently undergone large bursts of star formation. It should therefore be possible to see the rapid destruction of small stellar clusters in action. If this is indeed found by new observations, it will confirm our theory for the origin of globular clusters,"  Kruijssen added.

The simulations suggest that most of a globular cluster's traits were established when it formed. The fact that globular clusters are comparable everywhere then indicates that the environments in which they formed were very similar, regardless of the galaxy they currently reside in. In that case, Dr. Kruijssen believes, they can be used as fossils to shed more light on the conditions in which the first stars and galaxies were born.

The new work, led by Dr. Diederik Kruijssen of the Max Planck Institute for Astrophysics in Garching, Germany, appears in a paper in the journal Monthly Notices of the Royal Astronomical Society.

The Daily Galaxy via Royal Astronomical Society

Image credit: Southern African Large Telescope (SALT) project

error

"The Galaxy" in Your Inbox, Free, Daily