Andromeda, the Milky Way’s neighboring spiral galaxy, may be a cosmic cannibal that grew by swallowing smaller galaxies.
Astronomers came to this conclusion by observing a globular cluster of stars called the Dulais Structure, named after the Welsh term meaning “dark stream”, which now dwells in Andromeda but seems to come from outside galaxy. The Dulais structure is illuminated by star clusters orbiting like no other cluster in Andromeda, meaning the star stream could represent the “leftovers” of a galaxy-sized meal in the past. of Andromeda.
The findings support the idea that galactic growth occurs violently and sporadically, as large galaxies eat up their smaller counterparts.
“We have realized over the past few decades that galaxies grow by eating smaller systems– if few galaxies fall, they are eaten – that is galactic cannibalism,” said Geraint Lewis, astrophysicist at the University of Sydney and lead author of the new research, in a statement . “A few years ago we discovered that on the far outskirts of Andromeda there was a sign in the objects orbiting it that the galaxy had not grazed, but had eaten large quantities at two distinct periods.”
Related: The Andromeda Galaxy bears the scars of a catastrophic collision
If this new Andromeda growth pattern is correct, Lewis said, the next question is, exactly what spiral galaxyate these smaller galaxies.
“That then leads to the next question, well, what was actually consumed?” he said. “Because it doesn’t seem like a single thing; it seems like a collection of things that are all slowly being torn apart.”
Lewis and his colleagues found that Andromeda shows signs of two major feeding events in its past: one meal occurred within the last 5 billion years, and the other between 8 and 10 billion years ago. ‘years.
As the 13.8 billion-year-old galaxy aged, it got bigger, meaning that during those meals, matter in the universe would have been more densely packed than it is. is today. Since then, galaxies like Andromeda have formed and grown, transforming the universe from a wasteland of homogeneous matter into the feature-filled cosmos we see today – although the precision of this remains uncertain.
“We know that the universe was featureless at its birth in the big Bang and today it’s full of galaxies,” Lewis said. “Are these galaxies born fully formed or did they grow?
Studying galaxies like Andromeda helps astrophysicists better understand how galaxies evolve because seeing them from the outside provides a more complete picture than looking at the Milky Way from our position inside, the researchers said.
Is our galaxy also a cosmic cannibal?
Although there is evidence that our galaxy merged with other galaxies and even swallowed them, this is not yet settled our Milky Waygalaxy, which is similar in size and shape to Andromeda, has also engaged in bouts of galactic cannibalism to facilitate its own growth. However, the clear picture of feeding events and growth spurts in Andromeda shows that this is happening in the local universe.
“What this new result does is provide a clearer picture of how our local universe came together,” Lewis said. “This tells us that at least in one of the large galaxies there was this sporadic feeding of small galaxies. What we want to know is if the Milky Way did the same thing, or is it different ?”
Lewis and his team are now aiming to determine when the Andromeda feeding events occurred. This information will be important in perfecting galactic evolution patterns. To determine the dates of the galaxy’s meals, researchers must understand the distances at stake in Andromeda between its “local” features and the remains of other galaxies consumed. This will allow the team to recreate the history of Andromeda in three dimensions.
“That will then allow us to figure out the orbits, where things are going, and then we can start to roll back the clock and see if we can get this consistent picture of when things happened,” Lewis said.
The first indications of the origin of the Dulais structure as “remains” came from the work of two students: Tim Adams, from the University of Sydney, and Yuan Li, from the University of Auckland.
“When they come up to you and say, ‘I keep getting this signal, and it’s kind of weird,’ that’s when it gets really exciting,” Lewis said. “It opened a new door in terms of understanding. But exactly what it tells us – I think we have yet to work it out.”
The team’s research has been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society, and a preprint version is available via arXiv.
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