A “ghost from the past” recalls the infancy of the Milky Way

This globular cluster is in the last stages of its existence.
By | Published: December 11, 2015 | Last updated on May 18, 2023
Milky Way
The Milky Way arcs into a panorama in the southern sky, taken from the Paranal Observatory, Chile.
ESO/H.H. Heyer
Globular clusters are spherical-shaped or globular stellar groupings — hence the name — 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 situated around 30,000 light-years away in the southern constellation Chameleon. A team of Spanish and Italian astronomers has named it “a ghost from the Milky Way’s past” in an article published recently.

“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, 13,000 million years ago,” said Carlos de la Fuente Marcos, an independent astronomer who collaborates with colleagues from the Northern Catholic University and ESO in Chile, and the University of Padua in Italy.

E 3 is hidden behind younger and brighter objects located between the cluster and Earth, but it has been possible to analyze it thanks to the Very Large Telescope (VLT) in the European Southern Observatory (ESO) in Cerro Paranal, Chile. The data obtained 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,” said 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.”

Globular cluster E 3
The globular cluster E 3 (center).
According to another study on E 3 by Michigan State University (USA) researchers, this cluster is chemically homogeneous — it 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,” said De la Fuente Marcos. He explained that the study of these objects “enables us to gain insight into the infancy of the Milky Way.”

Native or captured?
Despite the recently published new data on this strange globular cluster, 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. Thousands of millions of years ago our galaxy cannibalized other smaller galaxies and kept their globular clusters. The rest were formed in-situ.

In the article, it is suggested that the object analyzed could be dynamically 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.

“We hope to obtain new data in 2016 thanks to more spectroscopic observations, and perhaps we will be able to give answers to these questions,” said De la Fuente Marcos.