doi.org/10.3847/2041-8213/adb433
Credibility: 989
#Galaxy
A stunning discovery on the outskirts of the Andromeda Galaxy is changing what we know about how galaxies form and evolve
Astronomers have found the faintest and smallest satellite galaxy ever seen, called Andromeda XXXV, and it’s making scientists rethink a lot of things.
This tiny galaxy defies expectations: It formed stars for billions of years longer than the satellites of our own Milky Way and survived a time when it should have been destroyed.
How did this happen? That’s the new mystery scientists are trying to solve.
A Small Galaxy with Big Impacts
A team led by the University of Michigan has made a discovery that challenges long-held ideas about galaxy formation, based on what we’ve learned from the Milky Way, our home in space.
The discovery comes from the edge of the Andromeda Galaxy, our closest neighbor among the large galaxies.
There, astronomers found the smallest and faintest satellite galaxy ever discovered around it.
Called Andromeda XXXV, this dwarf galaxy is about 3 million light-years away and is helping scientists better understand how galaxies change in different places and times in the universe.
Rewriting the History of Galaxies
This discovery raises more questions than answers, but according to Marcos Arias, the lead author of the study published on March 11 in the journal Astrophysical Journal Letters, this is normal when studying the universe.
He says that the cosmos still holds many secrets, but this galaxy helps correct what we know and shows how much we still have to learn.
“There is still a lot to discover,” says Arias, who did this work as an undergraduate student in the Department of Astronomy and is now a researcher in the field.
“We need to understand more about the formation, evolution and structure of galaxies near us before we can reconstruct the history of the universe and know how we got here.”
The Milky Way also has several satellite galaxies, or companions, which is why their story is still being written.
These companions are quite different from the main galaxy, but they are close enough to be pulled by its gravity.
And they are much, much smaller.
A Game-Changing Discovery
“These are full-fledged galaxies, but they are about a millionth the size of the Milky Way,” explains Eric Bell, a professor at the University of Michigan and a co-author of the study.
“It’s like a fully functioning human being, but the size of a grain of rice.”
Because they are so small, these satellite galaxies are much harder to see, because they shine so dimly.
Only in the last few decades have astronomers developed good enough technology to find most of the Milky Way’s satellites.
And, according to Bell, it is impossible today to see such faint galaxies orbiting systems more distant than Andromeda.
Until now, the Milky Way’s satellites have been our only source of information about these tiny galaxies.
Although Andromeda has had satellites found before-hence the name XXXV, or 35th-they were too big and bright to change what we know about the Milky Way.
“That’s why Marcos’ discovery is so important.
This type of galaxy has only been seen near the Milky Way before,” Bell says.
“Now we’ve been able to look at one in Andromeda, and it’s the first time we’ve done that outside our system.”
Using Hubble to See the Unseen
To find Andromeda XXXV, Arias combed through tons of observational data looking for signs of possible companion galaxies.
After shortlisting candidates, he and Bell got time on the Hubble Space Telescope to take a closer look.
“The chances of doing important research as an undergraduate in the astronomy department at the University of Michigan are huge,” Arias says.
The work was supported by the National Science Foundation and NASA.
With Hubble, they confirmed that Andromeda XXXV was a satellite galaxy and found that it was small enough to change some ideas about how galaxies evolve, such as how long it takes for them to form stars.
“It was really surprising,” Bell says.
“It’s the faintest thing we’ve found out there, so it’s a pretty interesting system.
But it’s also unexpected in a lot of ways.”
The team included scientists from a variety of institutions, including the University of Chicago, Utah Valley University, the Vatican Observatory, the University of La Serena in Chile, the University of Alabama, Montana State University, and the Leibniz Institute for Astrophysics in Potsdam, Germany.
A Cosmic Mystery
Bell explains that it’s not uncommon for astronomical ideas to get more complicated when we go beyond our own backyard.
When you only have one system to study, you can’t tell what’s common and what’s unique, he says.
Now, Andromeda XXXV has provided strong enough evidence to begin to separate these features.
The clearest difference between the Milky Way’s satellites and Andromeda’s is when they stopped forming stars.
The Secret of Andromeda’s Stars
“Most of the Milky Way’s satellites have very old populations of stars.
They stopped forming stars about 10 billion years ago,” explains Arias.
“In Andromeda, on the other hand, we see that similar satellites were able to form stars until about 6 billion years ago.”
This information helped researchers solve what Bell calls a “murder mystery” in Andromeda.
Both large and small galaxies need gas to form stars.
When the gas runs out, star formation stops.
The big question is: Does the gas run out on its own, or is it stolen by the larger galaxy nearby”
In the Milky Way’s satellites, the early end to star formation suggests that the gas ran out naturally.
In Andromeda, the smaller companions took longer to form stars, and it appears that the gas was “stolen.”
“It’s a bit dark, but it’s like: Did they fall in or were they pushed in? These galaxies seem to have been pushed in,” Bell says.
“We’re learning something new about how galaxies form.”
This extra star-forming time is even more intriguing when you consider the size of Andromeda XXXV and the history of the universe.
A Cosmic “Frying Pan”
In the beginning, the universe was super-hot and dense.
But as it reached its first billionth birthday, it expanded and cooled to the temperature of a cool spring day.
This temperature was perfect for the gas in the universe to clump together and form stars, which then clustered into galaxies.
But as the stars produced energy and black holes swallowed matter, the universe heated up again.
For small galaxies-those with less mass than about 100,000 suns-this was a death sentence for star formation.
The heat would fry the gas needed to create new stars.
“We thought they would all be fried, like the universe turned into a pan of hot oil,” Bell says.
But Andromeda XXXV was not fried.
“We thought it would lose all its gas, but that didn’t happen.
It’s only about 20,000 times the mass of the Sun, and yet it formed stars happily for another few billion years.”
How it survived is an unanswered question for now.
The Universe Always Surprises
“I don’t have an answer,” Bell admits.
“The universe did heat up, but we’re seeing that the consequences are more complicated than we thought.”
Organizations like NASA plan to launch missions in the coming years to find more satellite galaxies, which will help piece together the puzzle.
Arias and Bell are excited about this and want to learn as much as they can with the tools they have today, but they’re also comfortable with the unknown.
For Arias, that’s exactly what drew him to astronomy.
“It’s the universe,” he says.
“There’s always going to be something new to discover.”
Published in 03/12/2025 18h56
Text adapted by AI (ChatGPT / Gemini) and translated via Google API in the English version. Images from public image libraries or credits in the caption. Information about DOI, author and institution can be found in the body of the article.
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