In a feat of galactic archaeology, astronomers are using increasingly detailed information to trace the origins of our galaxy – and to learn how other galaxies formed in the early stages of the universe. Using powerful space telescopes such as Gaia and James Webb, astronomers can look back in time and look at some of the oldest stars and galaxies. Thanks to Gaia’s data on the positions and movements of stars in our Milky Way and Webb’s observations of early galaxies that formed when the universe was young, astronomers are learning how galaxies come together and have made surprising discoveries that indicate the early universe was busier and brighter . than anyone previously thought.
The earliest parts of the Milky Way
In a recent paper, researchers using the Gaia Space Telescope identified two streams of stars, called Shakti and Shiva, each containing a total mass of about 10 million suns and believed to have merged into the Milky Way about 12 billion years ago. .
These streams were present before the Milky Way had features like a disk or spiral arms, and researchers think they may have been among the earliest building blocks of the galaxy as it developed.
“What is very interesting is that we can detect these structures from such ancient times at all,” says lead researcher Khyati Malhan of the Max Planck Institute for Astronomy (MPIA). “These very massive fragments came in and collapsed under their own forces. gravity, and they actually formed the proto-Milky Way galaxy.”
This happened when the universe was young; the earliest galaxies only formed about 13 billion years ago. When these groups of stars came together to form the Milky Way, it is questionable whether the group they joined could be called a galaxy. While there are broad gravitational requirements for a given mass of stars to hold themselves together, there is no precise definition of when a group of stars can truly be called the beginning of a galaxy.
“When is a city a city?” said co-author Hans-Walter Rix, also from MPIA. “This is why there is no epoch when the galaxy formed. It has been an ongoing process.”
The Milky Way as a test case
Since there is still so much to learn about galaxy formation, it makes sense to start with our own Milky Way Galaxy as a test case. The Milky Way is “a huge average galaxy,” Rix said. Compared to the rest of the universe: “Half of the stars live in larger galaxies, half of the stars in smaller galaxies.”
What makes the Milky Way useful is that we have unique access to it, allowing us to see individual stars within it. That means researchers can identify large groups of stars that appear to have formed together with similar ages and levels of heavier elements. By looking at each of these groups, they can track how the galaxy was put together.
There are two main ways that stars enter galaxies. In the first case, large clouds of diffuse gas are present in an existing galaxy, and this gas condenses to form stars within it. Alternatively, stars that form in a satellite galaxy can be dragged into the main galaxy.
Today we most often see star formation in gas clouds; about 90 percent of the stars we see today formed this way. But in earlier stages of the universe, the potential for satellite accretion was much more important, because most stars from this period are thought to have formed in clumps that were then dragged into the young Milky Way.
To understand the history of the Milky Way, astronomers must trace the origins of these groups of stars and figure out what drew them into the galaxy we know today. “One of the big goals is, ‘can we reconstruct the early accretion events of the coming together of these pieces?’” Rix said.
Using Gaia data, the researchers were able to distinguish groups of stars with similar orbits that were located toward the center of the Milky Way. They are located about halfway between Earth and the galactic center and are found in the shape of a thick-walled torus orbiting the center of the galaxy.
The researchers suspect that the two star streams they discovered were some of the last pieces of the Milky Way to be absorbed during the satellite accretion phase, after which star formation in the galaxy became the main driver of stars joining the galaxy. “It seems like Shakti and Shiva may be the last hurray of that early phase, when it was all about bits and pieces coming together,” Rix said.