Beginnings of our Galaxy: Globular Clusters

The Milky Way arcs into a panorama in the southern sky, taken from the Paranal Observatory, Chile. Credit: ESO/H.H. Heyer

When our galaxy was born, ~13B yrs ago, a plethora of clusters containing millions of stars emerged. But over time, they have been disappearing. However, hidden behind younger stars that were formed later, some old and dying star clusters remain, eg E 3. European astronomers have now studied this testimony to the beginnings of our galaxy.

Globular clusters are spherical-shaped or globular stellar groupings which can contain millions of stars. There are about 200 of them in the Milky Way, but few are as intriguing to astronomers as the E 3 cluster. It is 30,000 light years away, in the southern constellation of Chameleon. “This globular cluster, and a few similar ones – such as Palomar 5 or Palomar 14 – are `ghosts´ because they appear to be in the last stages of their existence, and we say ´from the past´ because they are very old. They were formed when our galaxy was virtually new-born, 13 billion years ago,” says Carlos de la Fuente Marcos.

E 3 is hidden behind younger and brighter objects located between the cluster and Earth, but it has been possible to analyse it thanks to the Very Large Telescope (VLT) in the European Southern Observatory (ESO) in Cerro Paranal, Chile. The data revealed some surprises. “Unlike typical globular clusters, which contain hundreds of thousands and in some cases millions of stars, the object studied only has a few tens of thousands of them,” says De la Fuente Marcos. “Additionally, it doesn’t have the typical circular symmetry, but a much distorted, almost ghostly, rhomboidal shape, contorted by the galactic gravitational waves.”

This cluster is chemically homogeneous, ie doesn’t have several star populations in its interior. “This is characteristic of an object that was created in block, in one single episode, like what is supposed to have happened when our galaxy was born: very large star clusters (containing millions of stars) were formed, but what remains of them today are objects like E 3, ghosts from a distant past,” says De la Fuente Marcos. It “enables us to gain insight into the infancy of the Milky Way.”

Astronomers still have to clarify if it was really formed in our galaxy or not. It is known that some of its clusters are not native to the Milky Way, but were captured, even though they can currently be seen in its interior. Long ago, our galaxy cannibalised other smaller galaxies and kept their globular clusters. The rest were formed in-situ.

The object could be related to other clusters, such as 47 Tucanae, one of the richest and largest of the Milky Way. They could even share the same stream of stars. If this were the case, it would support the hypothesis that E 3 was captured in the distant past. http://www.agenciasinc.es/en/News/A-ghost-from-the-past-recalls-the-infancy-of-the-Milky-Way