The most relevant aspect of the discovery, however, is that this is the second time that a stellar system resembling a globular cluster, but “violating all the rules of its own category”, is found in the bulge of the Milky Way. A similar object, in fact, has been discovered 11 years ago by the same team of astrophysicists: Terzan 5 (this is its name) has been classified as a globular cluster for decades, but revealed the presence of both a stellar population as old as the Milky Way, and another one much younger (with an age of just 4.5 billion years) and with a much larger iron abundance. Its chemical composition is strikingly similar to that measured in Bulge stars, thus suggesting that this system is the fossil remnant of a much more massive structure that contributed to generate the central region of the Milky Way approximately 12 billion years ago.

While a single case could have been just a curious anomaly (usually, one swallow doesn’t make a summer!), the discovery that Liller 1 has very similar properties allowed the astrophysicists to define a new class of stellar systems that, so far, were disguised as globular clusters behind the thick dust clouds obscuring the central region of our galaxy: they have been named “Bulge Fossil Fragments”.

While observations of the distant Universe show that the central part (bulge) of spiral galaxies form through the merging of primordial massive clumps of gas and stars, no evidence of this process has  been found so far in the Milky Way. Thanks to a powerful synergy between the Gemini South telescope in Chile and the Hubble Space Telescope, a new class of stellar systems (named “Bulge Fossil Fragments”) has been discovered in the Mily Way bulge: they are the fossil relics of that assembling process. The discovery has been published in the Monday 14 December 2020 issue of the prestigious journal Nature Astronomy and it is the result of a study performed by an international team of astrophysicists led by Francesco Ferraro, professor at the Physics and Astronomy Department “Augusto Righi” of the Bologna University and associated to the Italian Astrophysics National Institute (INAF).

Redesigning the assembling history of the

Galactic Bulge

Artistic picture of Gemini South and HST looking at Liller 1 in the Milky Way bulge (Image Credit for the Milky Way panorama: ESO/S. Brunier). The HST optical images used in this study are the first ever acquired for Liller 1. Image Credit: F.R. Ferraro/C. Pallanca (UniBO)

The first optical image ever obtained for Liller 1 with the HST (left), and the infrared image acquired at the Gemini South telescope in Chile (right). Thanks to the combination of these extraordinary images, the true identity of Liller 1 has been finally unveiled: this system has been discovered to host two distinct stellar populations, a very old one (with an age of 12 billion years), and a much younger component, which has been generated just 1-2 billions years ago.

Image Credit: F.R. Ferraro/C. Pallanca (UniBO)

Image of the Milky Way in the direction of the galactic centre (Credit: ESO/S. Brunier), with the positions of the two Bulge Fossil Fragments identified so far (Liller 1 e Terzan 5) highlighted. The dark spots on light backgroud are thick dust clouds absorbing the light emitted by the sources behind them.   Image Credit: F.R. Ferraro/C. Pallanca (UniBO)

This is the reason why the team required the intervention of the Hubble Space Telescope, acquiring the first optical images of Liller 1: it could be hard to believe, but in 30 years of activity, Hubble never observed before this stellar system at optical wavelengths! Once combined together, the Gemini and HST images finally gave a clear picture of Liller 1, after the exclusion of the Galactic field intruders. The conclusion was indeed a big suprise: Liller 1 hosts at least two stellar populations with dramatically different ages, the oldest having formed about 12 billions years ago (that is to say, at the same epoch of the Milky Way formation), the second one being much younger, with an age of just 1-2 billion years!

Such a discovery unambiguously demonstrates that Liller 1 is not a globular cluster, but a much more complex system. It originated from a much more massive object that gave rise to a second generation of stars from gas chemically enriched by the older population, with a time delay of about 10 billion years between the two star formation bursts.

“As it happens to the archaeologists seeking for the vestiges of ancient civilizations, this discovery tells us that  we have to dig deeper into the Bulge of our galaxy. As in a sort of Rossetta Stone, the history of the Milky Way formation is written in these fossil remains, and their discovery is finally allowing us to read this story and redesign the scenario for the formation of the Bulge”, concludes Francesco Ferraro, full professor at the Bologna University and associated to the Italian Astrophysics National Institute (INAF), who led the reasearch and is first author of the scientific paper.

People involved in the project

University of Bologna: Francesco R. Ferraro, Cristina Pallanca, Barbara Lanzoni,  Chiara Crociati, Alessio Mucciarelli

INAF-OAS: Emanuele Dalessandro, Livia Origlia

University of California (USA): R. Michael Rich

Liverpool John Moores University (UK): Sara Saracino

European Southern Observatory (Germany): Elena Valenti,  Giacomo Beccari

University of Conception (Chile): Douglas Geisler, Sandro Villanova

Universidad Católica del Norte (Chile): Francesco Mauro, Cristian Moni Bidin

The discovery in a nutshell

Its name is Liller 1, and it islocated in the bulge of the Milky Way. For more than 40 years it has been catalogued as a genuine globular cluster, a system made of million stars with essentially the same age and the same chemical composition (at least in terms of the iron abundance), just one out of the many globular clusters (approximately 150) populating our own galaxy. A team of astrophysicists finally unveiled its true identity. A much more fascinating object was indeed hidden under the appearance of a globular cluster: it is a fossil fragment of one of the giant stellar clumps that contributed to form the central region (bulge) of the Milky Way through repeated mergers, about 12 billion years ago.

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In fact, altough it was known since decades, Liller 1 has been poorly studied so far because it is located in one of the most obscured regions of our galaxy, where star light is largely absorbed by thick clouds of interstellar dust. Because of these clouds, the visible light coming from those stars can be up to 10,000 times fainter than it is emitted! Only the infrared light is able to efficiently pass through those clouds. This is why the first inspection of Liller1 has been performed by using near-infrared images acquired with the Gemini South. This is a powerful telescope with an 8 m diameter mirror, equipped with instrumentation able to compensate for the distortion that the Earth atmosphere causes on stellar images. The data acquired with the Gemini South telescope are really extraordinary: thanks to an almost perfect correction from atmospheric distortions, they have an angular resolution comparable to that of the images acquired from space (where no degradative effects from the Earth atmosphere are present). Although they allowed the first, very detailed analysis of the stellar population of Liller 1, these images alone were not enough to provide the entire picture. In particular, they gave no answer to a crucial question: do the sampled stars all belong to Liller 1, or are there some “intruders”? Indeed, in the innermost regions of the Galactic bulge it is extremely difficult to distinguish between Liller 1 genuine members, and stars belonging to the “Galactic field”, that is to say, just observed along the same line of sight, but not being part of the system.

It is indeed an invaluable fossil, hailing all the way from the violent and chaotic formation epoch of most of the cosmic structures, also including the Milky Way that hosts our Solar System. Like archaeologists, who dig through the dust piled up on top of the vestiges of past civilizations, astrophysicists often have to deal with relics from remote cosmic times. In both cases, the correct “reading” of these remains is far from being obvious: understanding their true importance usually requires the detailed analysis of every trace imprinted on top of each relic, often through the use of different techniques and instruments. This is indeed what happend for Liller 1: unveiling its true identity has required the combination of images obtained with two of the most powerful telescopes available today, the Gemini South telescope in Chile, and the Hubble Space Telescope (HST) in orbit around our planet.

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.

Image Credit: F.R. Ferraro/C. Pallanca (UniBO)

Credit: ESO/S. Brunier