Scientists know that galaxies grow and evolve through collisions. But how can you decipher the family tree of the Milky Way, even after so many collisions over the past 12 or 13 billion years? An international group of astronomers seems to have found the answer and is looking at the globular clusters – swarms of ancient stars that surround our galaxy.
The Milky Way has about 150 of them, and many of them would have formed in the smaller galaxies that would eventually collide to form our current galactic home. The team of Diederik Kruijssen from Heidelberg University and Joel Pfeffer from Liverpool John Moores University believe that they used simulations to reconstruct much of the history of the Milky Way.
Her work, published in the Royal Astronomical Society’s Monthly Notices, is based on a series of simulations, e-mosaics, which provide a complete model for the formation, evolution and destruction of globular clusters.
Using simulations as models, they were able to relate the age, chemical composition, and orbital movements of globular clusters to the properties of their precursor galaxies, where they would have appeared more than 10 billion years ago. They then analyzed the globular clusters of the Milky Way themselves and were able to determine not only how many stars these precursor galaxies had, but also when they collided with the Milky Way.
Result: In the course of its history, our galaxy has cannibalized at least five other galaxies with more than 100 million stars and about 15 of them with at least 10 million stars. The larger precursor galaxies collided with the Milky Way 6 to 11 billion years ago. When evidence emerged from these ancient galaxies, they were given names: Kraken, Gaia-Enceladus, Sequoia, Helmi, and Sagittarius.
Genealogical tree of the Milky Way, built by Kruiijssen and colleagues, with five major precursors.
Almost immediately afterwards, a group led by the Brazilian astronomer Ricardo Schiavon of Liverpool John Moores University published in the same MNRAS the discovery of another fossil from one of the cannibalized galaxies – a previously unknown parent.
To do this, the researchers did not look at the globular clusters, but at the stars that populate the galactic halo, the spherical region that surrounds the disk of our galaxy. Using data collected by the Apogee experiment, which is part of the Sloan Digital Sky Survey (SDSS), the researchers indicated that much of it is a holdover from an ancient galaxy that was about 10 billion years ago Years ago collided with ours.
They named it Heracles – in honor of the legendary Greek hero. According to the researchers, the ancient galaxy Heracles is responsible for a third of all stars in the halo of our galaxy.
What these two studies are clearly beginning to draw is that the Milky Way had an atypical beginning, with many major collisions and a more turbulent life in the first billion years of its existence.
This column is published in Folha Corrida on Mondays.
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