In a feat of galactic archaeology, astronomers are using increasingly detailed information to trace the origins of our galaxy and learn how other galaxies formed in the early stages of the universe. Using powerful space telescopes like Gaia and James Webb, astronomers can peer back in time and examine 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 that the early universe was busier and brighter than anyone had previously imagined.
The earliest works 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 combined mass of about 10 million suns and believed to have merged into the Milky Way about 12 billion years ago .
These streams existed even before the Milky Way had features such as a disk or its spiral arms, and researchers believe they may be some of the earliest building blocks of the galaxy as it developed.
“What is very interesting is that we are able to detect these structures from such ancient times at all,” said lead researcher Khyati Malhan of the Max Planck Institute for Astronomy (MPIA). “These very massive fragments came and they collapsed under their own gravitational force and they actually formed the proto-Milky Way galaxy.
This happened when the universe was still young, with the earliest galaxies forming only about 13 billion years ago. When these groups of stars came together 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 for a certain mass of stars to hold 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 Ricks, also of the MPIA. “That’s why there is no era when the galaxy formed. It’s an ongoing process.”
The Milky Way as a test case
Since we still have 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 “an extremely medium-sized galaxy,” Ricks 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, providing the ability to see individual stars within it. This means that researchers can identify large groups of stars that appear to have formed together with similar ages and levels of heavier elements. Looking at each of these groups allows them to trace how the galaxy was assembled.
There are two main ways stars enter galaxies. In the first, large clouds of diffuse gas are present in an existing galaxy, and this gas condenses so that stars form within it. Alternatively, stars that form in a satellite galaxy can be dragged into the parent galaxy.
Today, we most often see star formation in clouds of gas, with about 90 percent of the stars we see today forming this way. But in the earlier stages of the universe, the option of accreting satellites was much more important, since most stars of this period are thought to have formed in clusters that were then dragged into the young Milky Way.
To understand the history of the Milky Way, astronomers must trace the origins of these clusters of stars and understand 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?'” Ricks said.
Using data from Gaia, the researchers were able to pick out groups of stars with similar orbits that were located toward the center of the galaxy. They are located about halfway between Earth and the galactic center and are in the form of a thick-walled torus orbiting the galactic center.
The researchers suspect that the two streams of stars they found are some of the last chunks of the Milky Way that were swallowed up during the accretion stage of satellites, after which star formation in the galaxy took over as the prime mover of stars. , joining the galaxy. “Shakti and Shiva appear to be perhaps the last hurrah of that early phase when mostly particles were collected,” Ricks said.
