How the Webb and Gaia missions provide new insight into galaxy formation

Enlarge / NASA’s James Webb Space Telescope reveals the Rho Ophiuchi cloud complex, the closest star-forming region to Earth.

In a feat of galactic archaeology, astronomers are using increasingly detailed information to trace the origin 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 go back in time and observe some of the oldest stars and galaxies. Between Gaia’s data on the position and movements of stars within our Milky Way and Webb’s observations of the first 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 more active and brighter. than anyone had previously imagined.

The first pieces 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 around 10 million suns and which are believed to have merged into the Milky Way. about 12 billion years ago. .

These streams were present even before the Milky Way had features like a disk or its spiral arms, and researchers believe they could 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,” said lead researcher Khyati Malhan of the Max Planck Institute for Astronomy (MPIA). “These very massive fragments arrived and collapsed under their own gravitational force, and basically formed the Milky Way protogalaxy.”

This happened when the Universe was still young, and the first galaxies formed only about 13 billion years ago. When these groups of stars joined together to form what would become the Milky Way, it is debatable whether the group they joined could have been called a galaxy. While there are broad gravitational requirements for a given mass of stars to be held together, there is no precise definition of when a group of stars can truly be considered the beginning of a galaxy.

“When is a city a city?” said co-author Hans-Walter Rix, also of the MPIA. “That’s why there is no epoch when the galaxy formed. “It has been an ongoing process.”

The Milky Way as a test case

With so much still to learn about galaxy formation, it makes sense to start with our own galaxy, the Milky Way, as a test case. The Milky Way is “a tremendously average galaxy,” Rix said. Compared to the rest of the Universe, “half of the stars live in larger galaxies, half of the stars live 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 originated together at similar ages and levels of heavier elements. Observing each of these groups allows them to trace how the galaxy was rebuilt.

There are two main ways that stars enter galaxies. In the first, there are large clouds of diffuse gas within an existing galaxy, and this gas condenses so that stars form inside it. Alternatively, stars that form in a satellite galaxy can be pulled into the main galaxy.

Today, we most often see star formation within gas clouds; About 90 percent of the stars we see today were formed this way. But in earlier stages of the Universe, the option of satellite accretion was much more important, since most stars of this period are believed to have formed in clumps that were then swept into the young Milky Way.

To understand the history of the Milky Way, astronomers need to trace the origin of these groups of stars and discover what attracted them to the galaxy we know today. “One of the big goals is ‘can we reconstruct the first accretion events of these pieces coming together?'” Rix said.

Using data from Gaia, the researchers were able to select groups of stars with similar orbits that were located towards the center of the galaxy. They are located approximately halfway between Earth and the galactic center and are in the shape of a thick-walled torus that rotates around the center of the galaxy.

The researchers suspect that the two streams of stars they discovered were some of the last pieces of the Milky Way to be absorbed during the satellite’s accretion stage, after which star formation within the galaxy took over as the main driver of the galaxies. stars joining the galaxy. “It seems like Shakti and Shiva are perhaps the last hurray of that early phase, when it was mostly fragments coming together,” Rix said.

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