Until today he was a suspect but had never been found. Our galaxy, the Milky Way, has a core of stars, among the very first it has ever hosted. And scientists have just proven its presence thanks to the latest data collected by the Gaia satellite!
At the top of its 100,000 light-year diameter, our galaxy, the Milky Way, is home to between 100 and 400 billion stars. Belonging to the barred spiral galaxies, it nevertheless contains a halo of stars around the galactic disk. The density of stars is lower there, but still present. Like most disc galaxies, it was primarily formed by collisions with other, smaller galaxies. She then sucked them up bit by bit until the two stars became one! But scientists have studied another part of the origin of our galaxy: what they call “galaxy core”, stars that would be located near the center and would originate from the very first moments of the Milky Way, less than a billion years after the Big Bang. The results of their survey have been pre-published on the server arXiv.
Two scenarios for the formation of the Milky Way
Composed of astronomers and astrophysicists, the team practices what is called galactic archaeology: scientists try to understand the history, the formation of galaxies, whether it is ours or those that surround it. But according to the study, two scenarios could lead to a galaxy with a disk as massive as ours. Two scenarios that often overlap. First, the researchers explain that a primitive core is formed, consisting of stars that are poor in metals. This is because at the beginning of the universe only the lightest elements, hydrogen and helium, were present. Then denser clouds, called protogalaxies, formed, and it was from them that the first stars, then galaxies, were born! Elements heavier than helium, referred to as “metals” in astrophysics, were first distributed in the interstellar medium after the first supernovae, the explosions of massive stars at the end of their lives. So according to the study, “Massive disk galaxies like our Milky Way should host an old, centrally concentrated, metal-poor stellar population.”
After the creation of this first nucleus, mergers between galaxies occur, which can also replace the formation of primitive stars: “in the context of the hierarchical formation of massive disc galaxies like the Milky Way, we would expect the oldest, metal-poor stars to form within one of the main overdensities that merged early to form the protogalaxy, or formed early into separate satellite galaxies which eventually merged with the main body The first channel is commonly referred to as in situ formation, the second accreted”. And the researchers’ goal was precisely to distinguish between these two scenarios!
According to them, it all started with a core in situ, then the creation of stars within a cloud of hydrogen and helium, then various collisions led to the expansion of the galaxy. In particular, the researchers mention a collision with the Gaia-Enceladus galaxy 11.5 billion years ago and several consecutive collisions with the dwarf satellite galaxy Sagittarius, still in orbit around the Milky Way. In addition to these two well-known events, the study points to other traces of an accretion-enlarged galaxy: “an increasing number of additional distinct accredited components of the Galaxy have been identified.”
18,000 stars found with very low metallicity
To find this mysterious core in situ, the researchers set out to track about 2 million giant stars, located less than 30° from the galactic center, in the direction of the constellation Sagittarius. They then set out to calculate their metallicity, based on the relative amount of iron in these stars compared to hydrogen, and compare it to the Sun. For this, they used the latest data collected by the Gaia satellite, revealed on July 12, 2022. And they found a cluster of 18,000 stars, both old, dating from almost 12.5 billion years ago, years, and poor in metal! All for a measured mass of 5.107 solar masses, but calculated at 108 solar masses taking dust obscuration into account, about 0.2% of the galaxy’s total mass.
Until now, old stars with poor metallicity have been found, such as SDSS J102915+172927, about 13 billion years old, or HE 1523-0901, the oldest in the galaxy, located in the galactic halo. But they are all isolated: this is the first time such an old cluster has been found! The study also explains that the stars found “ show no net rotation”, in contrast to stars of higher metallicity, which are ” dominated by rotation”. A result that all the more testifies to the age of these stars, formed when the galaxy was not yet one, and had not yet started its rotation!
For researchers, “All this information fits a picture where this metal-poor core of the Milky Way is the oldest proto-galactic component in our galaxy.” They conclude by explaining that their findings ” is in no way a new stellar component distinct from the Milky Way,” and “the spatial distribution of this population deserves to be modeled quantitatively”. Further studies will reveal more about this nucleus. in situ of the Milky Way, and so on its past!