Astronomers have unearthed the original core of the Milky Way

To this day it has been suspected but never found. Our galaxy, the Milky Way, has a core of stars, one of the very first it has ever hosted. And researchers 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. It is one of the barred spiral galaxies, yet contains a halo of stars around the galactic disc. There, the star density is lower, but still present. Like most disk galaxies, it was formed primarily through collisions with other, smaller galaxies. Then she sucked them up little by little until two stars became one! But researchers have been investigating another part of our galaxy’s origins: what they call the “Galaxy Core”, Stars that are near the center and would come from the very first moments of the Milky Way, less than a billion years after the Big Bang. The results of their study were pre-published on the server arXiv.

Two scenarios for the formation of the Milky Way

The team, made up of astronomers and astrophysicists, practices what is known as galactic archaeology: researchers try to understand the history of the formation of galaxies, be they ours or those around them. According to the study, however, two scenarios can 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 metal-poor stars. Because at the very beginning of the universe only the lightest elements were present, hydrogen and helium. Then denser clouds formed, called protogalaxies, and from them the first stars and then galaxies were born! Elements heavier than helium, known as “metals” in astrophysics, were distributed in the interstellar medium only 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 harbor an old, centrally concentrated, metal-poor stellar population.”

After the formation of this first core, mergers between galaxies occur, which could also replace the formation of primitive stars: “In the context of the hierarchical formation of massive disk galaxies such as the Milky Way, we would expect that the oldest, metal-poor stars formed within one of the main superdensities that merged early into the protogalaxy, or formed early into separate satellite galaxies that eventually merged with the main body. The first channel is commonly referred to as the in situ formation, the second as the accretion”. And it was the goal of the researchers to differentiate precisely between these two scenarios!

According to her, it all started with a core on site, i.e. the formation of stars in a cloud of hydrogen and helium, then various collisions led to the enlargement 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, which is still in orbit around the Milky Way. In addition to these two well-known events, the study points to other evidence of an accreted galaxy: “An increasing number of additional distinct accretion components of the galaxy have been identified.”

Found 18,000 stars with very low metallicity

To find that mysterious core on siteThe researchers tracked about 2 million giant stars located less than 30° from the galactic center towards the constellation Sagittarius. Then they set out to calculate their metallicity, based on the relative amount of iron in those stars compared to hydrogen, and compared it to the Sun. For this, they used the latest data collected by the Gaia satellite, revealed on July 12, 2022. All for a measured mass of 5.107 Solar masses, but calculated as 108th Solar masses accounting for dust obscuration, about 0.2% of the galaxy’s total mass.

Old stars with low metallicity have been found so far, such as SDSS J102915+172927, around 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 ” dominated by rotation”. A result that is all the more testament to the age of these stars, which formed before the galaxy was one and had not yet started its rotation!

So for researchers “All of this information fits a picture in which this metal-poor core of the Milky Way is the oldest protogalactic component of our galaxy.” Finally, they state that their results “ are 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 core. on site the Milky Way and so on their past!

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