The Milky Way swallowed up another galaxy a quarter of its mass, 10 billion years ago, in a “violent collision.”
The newborn universe (about 13.7 billion years ago) was very different from the universe we know today, despite the fact that, when we observe the tiny portion that we can study from Earth, what we do is see them as they were millions of years ago in space-time.
In the early Milky Way, stars are believed to first form at high speed, forming the first dwarf galaxies, whose mergers gave rise to the most massive galaxies today, including our own.
However, the exact chain of events that produced the Milky Way was not known. Up to now.
The exact measurements of the position, brightness and distance of around a million stars in our galaxy within a radius of 6,500 light years from the Sun, obtained with the Gaia space telescope, have allowed a team from the Instituto de Astrofísica de Canarias (IAC) reveal some of its early stages, to conclude that the Milky Way swallowed up another galaxy a quarter of its mass, 10 billion years ago, in a “violent collision.”
Carme Gallart, IAC researcher and first author of this article, which has been published in the journal Nature Astronomy , explains it like this: “We have analyzed, and compared with theoretical models, the distribution of colors and magnitudes (brightness) of stars in the Milky Way, dividing them into several components: the so-called stellar halo (a spherical structure that surrounds spiral galaxies) and the thick disk (stars that make up the disk of our galaxy, but occupy a certain height range) “.
And he adds: “The novelty of our work is that we have been able to assign precise ages to the stars that belong to the galaxies that merged and, knowing these ages, when the merger occurred .”
First days of the Milky Way, art print. Credit: Gabriel Pérez Díaz, SMM (IAC).
But if both components were formed at the same time , what difference does one make from the other?
The final piece of the puzzle was given by the amount of metals (elements that are not hydrogen or helium) in the stars of one or another component, as explained by Tomás Ruiz Lara, a researcher at the IAC and another of the authors of the article: “The stars in the blue component they have a smaller amount of metals than those of the red component “.
They identified two distinct star clusters: a “blue” one containing less metal, and a “red” one containing more metal.
The early Milky Way ‘swallowed’ another smaller galaxy long ago
After studying their motion and composition, the team determined that both sets of stars were equally old, but the bluer ones had settled into “chaotic motion,” proof that the Milky Way had swallowed up a smaller galaxy in a far corner of time.
They also determined that the collision contributed to “violent bursts” of star formation for another four billion years, after which gas from those formations settled into the thin disk of the Milky Way that runs through the center of the galaxy .
Previous studies had found that the galactic halo showed clear signs of being made up of two distinct stellar components, one dominated by stars that are bluer than the other. The movement of the stars in the blue component quickly allowed it to be identified as the remnants of a dwarf galaxy (Gaia-Enceladus) that impacted the early Milky Way. However, the nature of the red population and the time of the merger between Gaia-Enceladus and our Galaxy were unknown until now.
These findings, along with predictions from the simulations that are also discussed in the article, have allowed the researchers to complete, for the first time, the history of the formation of the Milky Way.
The Milky Way contains at least 100 billion stars and its center contains an intense radio source believed to be the supermassive black hole Sagittarius A.
