The primordial history of the Milky Way and its fossil discoveries

As archaeologists dig in the hope of finding traces of the past, an international group of astrophysicists managed to penetrate the thick cloud of dust in the center of the Milky Way (also known as the bomb), discovering primordial gas lands and hitherto unseen stars. . They called this new class of stellar system “Bombarded Fossil Fragments.” A team of researchers led by Francesco Ferraro (Department of Physics and Astronomy “Augusto Righi” at the University of Bologna and member of the National Institute of Astrophysics – INAF) conducted a study published in Nature Astronomy.

The researchers learned about this new class while analyzing Liller 1. The latter is a stellar system in the Milky Way that has been classified as a “globular cluster” for more than 40 years. of stars of the same age. (The Milky Way has at least 150 globular groups). However, researchers took a close look at Liller 1 and found that its real identity is actually more fascinating than previously thought. Indeed, Liller 1 is a fossil fragment of one of the gigantic star clusters that, about 12 billion years ago, merged to form the central region (swelling) of the Milky Way.

“Our results clearly show that Liller 1 is not a globular cluster, but a much more complex object,” says Professor Francesco Ferraro, the study’s first author and coordinator. “It’s a stellar relic, a fossil find that contains the history of the Milky Way.”

A validation result

The existence of “cosmic discoveries” had already been suggested when researchers discovered a similar object, Terzan 5, a few years ago. Terzan 5 looked like a globular cluster inside our galaxy bomb, but on closer inspection, its features were not consistent with those of other globular clusters.

However, an isolated case is only an interesting anomaly. This is why Liller 1 is so important. The common features of Terzan 5 and Liller 1 confirm the existence of a new class of star systems not identified to date.

Fossil fragments

What are the characteristics of the Bulge fossil fragments? These objects are disguised in globular groups, but they are fundamentally different if we look at the age of the stars that make them up. Two stellar populations are in these systems: one is as old as the Milky Way – formed 12 billion years ago – and the other is much younger. On the one hand, this shows that these stellar systems appeared during the early stages of the Milky Way’s formation; on the other hand, they demonstrate that they are capable of generating multiple stellar events.

“The features of the stellar populations Liller 1 and Terzan 5 suggest that both systems formed at the same time as the Milky Way,” explains one of the study’s authors, Barbara Lanzoni, a professor at the University of Bologna and a member of INAF. “Younger stellar populations are richer in iron and tend to cluster in the central areas of the bomb. Indeed, this is consistent with a context of self-enrichment in which the gas emitted by older stars forms new ones.”

Beyond the clouds

Reaching these discoveries was everything, but easy. Liller 1 is located in one of the most hidden regions of our galaxy, where thick clouds of interstellar dust dim the starlight, making it up to 10,000 times weaker. The only way to get through these clouds is infrared light. This is why the researchers chose Gemini South to perform the Liller 1 inspection. Gemini South is a powerful telescope with a diameter of 8 meters capable of compensating for distortions in stellar images caused by the Earth’s atmosphere.

The clarity of the images of the Southern Gemini is unmatched. Thanks to these incredible images, researchers could make a detailed preliminary analysis of the Liller 1 stellar population. Despite this preliminary analysis, the researchers still had something to do to get a complete picture of the composition of this stellar system. Indeed, they needed to know if all the stars in those images belonged to Liller 1 or if some of them were simply in the same line of sight, but they did not belong to him. They were able to solve this problem by using additional observations made through the Hubble Space Telescope.

“After combining the two sets of images, we removed the stars that did not belong to Liller 1 and finally had a clear and detailed picture of this stellar system,” says Cristina Pallanca, a researcher at the University of Bologna and a member of INAF. who co-authored the study. “Our results surprised us: Liller 1 is home to at least two dramatically different stellar populations, the oldest of which formed about 12 billion years ago, at the same time as the Milky Way; the second, much younger, only 1-2 billion years ago. “

A discovery that is extremely similar to what they learned about Terzan 5, which similarly hosts a stellar population as old as the Milky Way and a much younger one (4.5 billion years).

“The discovery that Liller 1 and Terzan 5 share very similar characteristics allowed the identification of a new class of stellar systems from some ancestors that were massive enough to retain the gas emitted by supernovae. What I noticed are just a few fragments of these massifs. structures “, adds Emanuele Dalessandro, researcher at INAF – Space Science Observatory (OAS) in Bologna and co-author of the study.

This then confirmed the existence of the Bulge Fossil Fragments, ie stellar systems composed of relics of massive primordial objects that, 12 billion years ago, gave birth to the Milky Way.

“The history of the Milky Way is written in these fossil remains. The latter are the signs of an age when the universe was very young, only 1 billion years old,” concludes Professor Ferraro. “It simply came to our notice then. Thanks to the discovery of these fossil remains, we can begin to read the history of the Milky Way and perhaps redefine our knowledge about the formation of the bomb. ”

“A new class of fossil fragments from the hierarchical assembly of the galactic bomb”, is the title of this study published in Nature Astronomy.

Provided by the University of Bologna