The researchers analyzed data from the most recent release of the ESA mission.
How does space archeology work?
Our galaxy, the Milky Way, is graduallywas formed over almost the entire history of the Universe, which covers 13 billion years. Over the past decades, astronomers have been able to reconstruct different eras of galactic history. In the same way as archaeologists would restore the history of the city. For example, according to the exact dates of construction of some buildings.
Also, when reconstructing cities, archaeologists usuallythey study, for example, more primitive building materials or construction styles. This is how they understand that some objects existed before others. Scientists understand when the remains of old buildings are underneath other (and therefore newer) buildings. Last but not least, spatial patterns are important: many cities have a central “old town” surrounded by districts that emerged much later.
For galaxies, and, in particular, for the MilkyWays, space archeology works in a similar way. The main building blocks of a galaxy are its stars. For a small subset of stars, astronomers can determine exactly how old they are. For example, this works for so-called subgiants—a short phase of stellar evolution when the brightness and temperature of an object can be used to determine its age.
Chemical "age detector"
More generally, almost all starsThey have their own “building style” that helps determine their age. It's about metallicity. It is defined as the amount of chemical elements heavier than helium contained in the “atmosphere” of a star. These "metals" are produced within stars by nuclear fusion and are released shortly before or at the end of the star's life—some when the "atmosphere" of a low-mass star dissipates, heavier elements with greater force when the star has a low mass. ;with a large mass it will explode as a supernova. Thus, each generation of stars “seeds” interstellar gas, from which the next is formed. As a rule, each of them has a higher metallicity than other generations.
Map of the Milky Way. Photo: NASA
As for large-scale structures, howand in the “ancient city”, spatial distribution matters. A galaxy is less static than a city—“space buildings” usually don’t move, unlike stars. Given this, movement patterns also encode important information.
Thus, the stars of the Milky Way may be limitedcentral regions, or they may be part of an orderly rotational motion in the thin or thick disk of the Milky Way. Or they may become part of the chaotic disorder of the orbits of the expanded halo of stars in the Galaxy, including very eccentric ones, which constantly pass through the inner and outermost regions.
How big galaxies grow
Galaxies formed by mergersand collisions of small protogalaxies, as well as huge amounts of fresh hydrogen gas. This is the raw material that spreads over billions of years to create new stars.
A star disk appears after a collisionprotogalaxies, when another protogalaxy is attached to the resulting object. It must fly far enough from the center (“large orbital angular momentum”). But when two sufficiently large galaxies merge (in a “big merger”), their gas reservoirs will heat up, forming a complex elliptical galaxy.
Hypothetical collision of the Milky Way and Andromeda. Photo: NASA
Reconstruction of this kind of history isa question of combining increasingly informative observations with increasingly complex modeling. And while the general picture of what happens when galaxies form and evolve has been around for decades, the specifics have emerged relatively recently, thanks in large part to research that has yielded better and more complete data.
How Scientists Studied the Milky Way's Teenage Years
To study the "adolescent period" of developmentMilky Way, the researchers used data from ESA's Gaia satellite and the LAMOST spectral survey. The goal is to determine the ages of 250,000 subgiant stars. Through analysis, astronomers have reconstructed the consequences of the Milky Way's exciting period 11 billion years ago and its subsequent calmer, "adult" life.
Then astronomers noticed that the oldest starsin the “teenage sample” they were already distinguished by significant metallicity, about 10% more than that of the Sun. It is clear that before the formation of these stars, there must have been even earlier generations of stars that “polluted” the interstellar medium with metals.
What modeling says about the Milky Way's ancient core
Existence of earlier generationsconsistent with predictions from cosmic history simulations. Thus, scientists have confirmed that the Milky Way appeared from the merger of three or four protogalaxies, which formed in close proximity to each other, and then merged. As a result, their stars were located in the form of a relatively compact core—several thousand light years in diameter. By cosmic standards, this is quite a bit for the galaxy.
Center of the Milky Way. Photo: NASA
And, in addition, the simulations predicted where it would be possiblefind “surviving” old stars. Scientists have suggested that they are “hiding” in a compact core, which they called the “ancient heart” of the Milky Way. It is located in and near the central regions of our galaxy even today, billions of years later.
How scientists searched for ancient stars
To study them in detail, scientists usednew observation methods. The LAMOST telescope, which was used in the previous study, due to its location on Earth and the inability to conduct observations in the summer months (due to monsoons), cannot observe the main regions of the Milky Way at all. And the subgiants are too dim to be observed at a distance of about 7,000 light years, making the central regions of the Galaxy completely inaccessible.
Let us remind you that in in addition to those rareFor stars that can be used to determine a specific age, there is a more general indicator of stellar metallicity—“different styles of structure” that allow stars to be sorted into older and younger ones.
The third data release was released in June 2022ESA Gaia mission. Since 2014, it has been measuring highly precise position and motion parameters, including distances, of more than a billion stars. The third issue was the first to include data on the spectrum of 220 million astronomical objects.
A map of particularly metal-poor giant stars,determined from the third release of Gaia DR3 data, which shows in a concentrated area (circled) the stars of the “poor old heart” of the Milky Way galaxy. The map shows the entire night sky in the same way that some world maps show the surface of the Earth. In the center of the map the direction towards the center of our galaxy is indicated. Photo: H.-V. Ricks/MPIA
Thanks to spectra, astronomers obtain informationabout the chemical composition of the star's atmosphere, including its metallicity. Additional analysis was required to “extract” reliable metallicity values from the Gaia data.
Scientists paid attention specifically to redgiant stars. They are about a hundred times brighter than subgiants, and they are easy to observe even in distant regions of the galactic core. They also have spectral features that encode their metallicity and are easy to detect.
For the analysis itself, astronomers turned toto machine learning methods. They pre-trained their neural network and used a special algorithm. As a result, the researchers gained access to a sample of precise data on the metallicity of 2 million giant stars in the inner part of the galaxy.
What did the scientists find out?
The data helped them scientists relatively easilyidentify a population of stars at the galactic center, which scientists have dubbed the "poor old heart" of the Milky Way, given their low metallicity, estimated age and central location. The distances provided by Gaia (using the parallax method) allow for a 3D reconstruction. According to it, these stars are located within a relatively small region around the center, approximately 30,000 light years across. And this “heart” is approximately 12.5 billion years old.
The researchers also found that adolescenceThe Milky Way coincided with the last billion years of influence with another galaxy - Gaia Enceladus / Sausage, the remains of which were discovered in 2018. This triggered a phase of intense star formation and resulted in the comparatively thick disk of stars that can be observed today. In general, the maturation of the Milky Way has consisted of a moderate influx of hydrogen gas, which is deposited in the extended thin disk of our galaxy, with the slow but continuous formation of new stars over billions of years.
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Cover photo: rare-gallery.com