In a feat of galactic archaeology, astronomers are tracing the origins of our galaxy with more detail than ever before and learning how other galaxies formed in the early universe. Using powerful space telescopes such as Gaia and James Webb, astronomers can reach back in time to observe the oldest stars and galaxies. From Gaia’s data on the positions and motions of stars in the 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 making surprising discoveries that suggest the early universe was more busy and bright than anyone had ever imagined.
The oldest part of the Milky Way galaxy
In a recent paper, researchers using the Gaia Space Telescope found that Two Star StreamsThe two galaxies, called Shakti and Shiva, each have a mass equivalent to about 10 million suns, and are thought to have merged into the Milky Way about 12 billion years ago.
These flows existed before the Milky Way developed features like a disk and spiral arms, and researchers believe they may be some of the earliest building blocks in the development of a galaxy.
“What’s really exciting is that we can detect these structures from so long ago,” said lead researcher Kathi Mulhan of the Max Planck Institute for Astronomy (MPIA). “These very massive pieces came flying in, collapsed under their own gravity, and basically formed the proto-Milky Way.”
This happened when the Universe was still young, with the first galaxies forming around 13 billion years ago. When these groups of stars came together to form the Milky Way, it’s debatable whether they could be called a galaxy. There is a wide range of gravitational conditions that a certain amount of stars need to come together, but 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?” says co-author Hans-Walter Ricks, also from MPIA. “That’s why there is no epoch when galaxies formed. It’s been a continuous process.”
The Milky Way as a test case
There’s still a lot to learn about galaxy formation, so it makes sense to start with our own Milky Way as a test case. The Milky Way is a “very average galaxy,” Ricks says. Compared to the rest of the universe, “half of the stars live in larger galaxies, and the other half live in smaller galaxies.”
The Milky Way is useful because we have unique access to it and can look at individual stars within it. This means researchers can identify groups of large stars that seem to have been born at the same time, with similar ages and levels of heavy elements. By observing each of these groups, they can trace how the galaxy came together.
There are two main ways that stars enter a galaxy: first, there is a large cloud of diffuse gas within an existing galaxy, and this gas condenses and stars form within it, and second, stars form in a satellite galaxy and are drawn into the host galaxy.
Today, star formation as we see it most often occurs in gas clouds – about 90 percent of the stars we see today form in this way – but the moon accretion option was much more important in the early Universe, since most stars at this time are thought to have formed as clumps 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 star clusters and figure out what attracted them into the galaxy we know today. “One of the big goals is, ‘Can we reconstruct the early accretion events that brought all these pieces together?'” Ricks says.
Using Gaia data, the researchers were able to single out a group of stars with similar orbits that are located towards the center of the galaxy. These stars are found roughly halfway between Earth and the center of the galaxy, in a thick-walled torus shape that orbits the galaxy’s center.
The researchers speculate that the two stellar streams they found may have been some of the Milky Way’s final debris absorbed during the moon accretion phase, and that star formation within the galaxy was then the main driving force behind stars joining the galaxy. “Shakti and Shiva look like the final upsurge of an earlier phase when the fragments were mainly coming together,” Ricks said.