In galactic archaeology, astronomers use 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 like Gaia and James Webb, astronomers are able to peer back in time to look at some of the oldest stars and galaxies. Between Gaia’s data on the position and motion of stars in our Milky Way and Webb’s observations of early galaxies that formed when the universe was still young, astronomers are learning how galaxies come together and have made surprising discoveries that suggest the early universe was busier and more clear. than anyone had previously imagined.
The earliest pieces of the Milky Way
In a recent paper, scientists using the Gaia space telescope identified two streams of stars, named 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 elements like a disk or its spiral arms, and researchers believe they could be some of the first building blocks of the galaxy as it evolved.
“It is very interesting that we are able to detect these structures at all from such ancient times,” said lead researcher Khyati Malhan of the Max Planck Institute for Astronomy (MPIA). gravitational force and essentially therefore created the Milky Way galaxy.
This happened when the universe was still young, with the oldest galaxies forming only about 13 billion years ago. When these groups of stars merged to form what would be the Milky Way, it is debatable whether the group they joined could even be called a galaxy. Although there are broad gravitational requirements to hold a certain number of stars 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 of MPIA. “This is why there is no epoch when the galaxy formed. It’s been an ongoing process.”
The Milky Way as a test case
Since we still have a lot 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 hugely average galaxy,” Rix said. Compared to the rest of the universe: “Half the stars live in larger galaxies, half the stars live in smaller galaxies.”
What makes the Milky Way useful is that we have unique access to it and the ability to see individual stars. This means that scientists can identify large groups of stars that appear to form together with similar ages and levels of heavier elements. Looking at each of these groups allows them to see how the galaxy was put together.
Stars enter galaxies in two main ways. In the first, large clouds of diffuse gas are present in an existing galaxy, and this gas condenses to form stars. 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, with roughly 90 percent of the stars we see today formed this way. But in the earlier stages of the universe, the possibility of satellite accretion was much more important, since most stars from this period are thought to have formed in clusters that were then pulled into the young Milky Way.
To understand the history of the Milky Way, astronomers need to trace the origins of these groups of stars and find out what drew them into the galaxy we know today. “One of the big goals is ‘can we reconstruct the early accretion events of these pieces coming together?'” Rix said.
Using the Gaia data, scientists were able to pick out groups of stars with similar orbits that were located toward the center of the galaxy. Located about halfway between Earth and the galactic center, they are in a thick-walled torus shape that revolves around the center of the galaxy.
The researchers believe that the two streams of stars they discovered were some of the last pieces of the Milky Way that were absorbed during the satellite accretion phase, after which star formation in the galaxy became the primary driver of stars joining the galaxy. “It looks like Shakti and Shiva might be the last hurray at that early stage when it was mostly bits and pieces coming together,” Rix said.