History Written In The Stars

“The history of the Milky Way is written in these fossil remains. The latter are tokens of an age during which the Universe was very young, just 1 billion years old.” Professor Francesco Ferraro

Archaeologists and historians dig into the past of human habitation on Earth. We are but a tiny speck in a wondrous Universe.

Astrophysicists delve into the deep past of Space. An international group of scientists led by Francesco Ferraro (Department of Physics and Astronomy “Augusto Righi” at the University of Bologna has been examining a thick cloud of dust around the centre of the Milky Way – ‘The Bulge’.

They discovered clumps of gas and stars while analyzing Liller 1, which they have called ‘Bulge Fossil Fragments.’

Panoramic view of the Milky Way (Credit: ESO/S. Brunier) with the location of the two Bulge Fossil Fragments discovered so far (Liller 1 and Terzan 5) highlighted. Credit: F. R. Ferraro / C. Pallanca (University of Bologna)

Liller 1 is a stellar system in the Milky Way bulge that for more than 40 years has been classified as a “globular cluster”, i.e. a system composed of millions of same-aged stars (the Milky Way has at least 150 globular clusters). However, researchers observed Liller 1 closely and found out that its real identity is actually more fascinating than so far believed. Indeed, Liller 1 is a fossil fragment of one of the giant stellar clumps that, approximately 12 billion years ago, merged to form the central region (bulge) of the Milky Way.

Professor Francesco Ferraro, explained:

“Our results clearly show that Liller 1 is not a globular cluster, but a much more complex object.

“It is a stellar relic, a fossil finding that contains the history of the Milky Way formation”.

Terzan 5

The existence of “cosmic findings” had already been suggested when researchers discovered a similar object, Terzan 5, some years ago. Terzan 5 looked like a globular cluster within our galaxy bulge, but, at a closer analysis, its features were not consistent with those of other globular clusters.

However, an isolated case is just an intriguing anomaly. This is why Liller 1 is so important. Terzan 5 and Liller 1 shared features confirm the existence of a new class of stellar systems unidentified until today.

Bulge Fossil Fragments

These objects are disguised as globular clusters, but are fundamentally different, if one looks at the age of the stars composing them. Two stellar populations are in these systems: one is as old as the Milky Way – it formed 12 billion years ago – and the other one is much younger. On the one hand, this shows that these stellar systems appeared during the Milky Way early stages of formation; on the other hand, it demonstrates that they are able to engender multiple events of stellar generation.

Professor Barbara Lanzoni, added:

“The features of Liller 1 and Terzan 5 stellar populations suggest that both systems formed at the same time of the Milky Way.

“Younger stellar populations are richer in iron and tend to cluster in the central areas of the bulge. Indeed, this is in line with a context of self-enrichment in which the gas ejected by older stars forms new ones”.

Gemini South

Liller 1 is located in one of the most obscured regions of our galaxy, where thick clouds of interstellar dust dim starlight making it up to 10,000 times fainter. The only way of getting through these clouds is infrared light.

The researchers used Gemini South which is a powerful telescope with a diameter of 8 meters able to compensate for the distortions in stellar images caused by the atmosphere of the Earth.

The sharpness of Gemini South images is unparalleled. Thanks to these incredible pictures, researchers could do a detailed preliminary analysis of Liller 1 stellar population.

Despite this preliminary analysis, researchers had still some work to do to have a complete picture of the composition of this stellar system. Indeed, they needed to know if all the stars shown by those images belonged to Liller 1, or if some of them were simply in the same line of sight, but did not belong to it. They managed to solve this issue by resorting to further observations performed through the Hubble Space Telescope.

Cristina Pallanca, described the process:

“After having combined 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.

“Our results surprised us: Liller 1 hosts at least two stellar populations with dramatically different ages, the oldest having formed about 12 billion years ago, the same time the Milky Way formed; the second one, much younger, having formed just 1-2 billion years ago”.

A discovery that is remarkably similar to what they found out about Terzan 5, which similarly hosts one 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 features allowed for the identification of a new class of stellar systems originated from some ancestors that were massive enough to retain the gas ejected by supernovas. What we observed are just some fragments of these massive structures”, added Emanuele Dalessandro.

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

Professor Ferraro summed it up:

“The history of the Milky Way is written in these fossil remains. The latter are tokens of an age during which the Universe was very young, just 1 billion years old.

“Now we need to go deeper. Thanks to the discovery of these fossil remains we can start reading the history of the Milky Way and maybe re-define our knowledge about the formation of the bulge”.

The research was published in Nature Astronomy

Related article: The Sky Above You – January 2021

Categories: Science

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