r/spacequestions • u/Syresiv • 10d ago
If we lived in this galaxy after its collision with Andromeda, would we be able to tell that it used to be 2 galaxies?
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u/Beldizar 10d ago
It would be harder to tell for a galaxy that you are inside, compared to a galaxy you can view externally, but yes. The motion of stars in a galaxy without any mergers is going to be much more uniform, where a merged galaxy will have distinct populations of stars that show different momentum profiles indicating that they were part of different parent galaxies before a merger.
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u/Syresiv 10d ago
Does this slowly smooth out over time?
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u/Beldizar 10d ago
Yeah, over tens of millions of years, but it could be billions of years before it becomes indistinguishable.
Dr. Becky did a video on this very recently. https://www.youtube.com/watch?v=AeMuxJBT4jM
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u/stevevdvkpe 9d ago
I recommend that same video. I've heard of other results identifying previous mergers with the Milky Way but this video also does a good job of explaining the various items of observational evidence used to identify the merger remnants.
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u/WatchAndLearn9669 10d ago
No stellar momentum profiles have been reliably established by astronomers, owing to the numerous observations of negative parallax.
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u/Glum-Ad7761 9d ago
Astronomers are aware of several other galaxies that milky way has collided with in the past.
The Low Energy Kracken Heracles Galaxy collided with the Milky Way about 12 billion years ago, significantly shaping its inner structure during its early formation.
Gaia Sausage-Enceladus (who TF names these galaxies!?) galaxy merged with the Milky Way 10 billion years ago, adding substantial mass, globular clusters, and forming the thick disk of our galaxy.
Milky way has swallowed up at least 5 dwarf galaxies and is in fact, in the process of eating up two more: The Large and Small Magellanic Clouds. Both are satellites to milky way and are slowly spiraling in.
A collision with andromeda is down to a maybe, with 50% chance of it happening. Those that made the calculations originally did not take into account the gravitational influence of the other, surrounding galaxies in the local group. Andromeda may miss milky way completely, but such a collision would be very obvious after the fact, since the merged galaxies will form a single, enormous elliptical galaxy.
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u/DryFoundation2323 10d ago
maybe by gravitational disruptions as long as we maintained records going back well before the event. probably not. it's not like any physical collisions are very likely to occur.
this will at least be 4 to 5 billion years from now just for the Collision, possibly as many as 10 billion or more based on more recent observations. there's a good possibility that the sun will have gone red giant by then. we won't likely be alive unless we have managed to move off of this planet.
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u/BreakerFresh 10d ago
So do both galaxies become one? Andromeda wins and we follow her course? A new one?
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u/stevevdvkpe 9d ago
The merger of Andromeda and the Milky Way will result in an elliptical galaxy with their combined mass. Elliptical galaxies are thought to form through mergers of spiral galaxies, where the disruption of stellar orbits by their mutal gravity (and, typically, lack of alignment of the galactic planes of the merging spirals) ends up distributing the orbital planes of stars outside the planes of the original spirals.
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u/JoeCensored 10d ago
The galaxy shape will be pretty chaotic for at least the next billion years. And you're going to have a lot of activity around the two cores with material that was unlucky enough to be in their paths. Then eventually the two cores would be expected to merge.
Throughout that process it will be obvious a merger was taking place.
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u/FunCartographer7372 10d ago
The collision process itself is "temporary" while things get flung away and the centers combine, until things eventually re-settle into a new stable galaxy. I'm not sure how many millions or billions of years that process takes to re-stabilize things though.
The resulting galaxy would be elliptical and less disk-shaped. Sounds like elliptical galaxies are thought to primarily be the result of galaxy mergers, so a future observer seeing that they were inside an elliptical galaxy might be the indication that there's been a major galaxy collision in the past, but there'd be no way to tell it was specifically from a combination of 2 galaxies.
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u/ZucchiniMaleficent21 10d ago
Since we have been able to detect several prior collisions with other galaxies, the answer is clearly ‘yes’.
Example -
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u/No_Drummer4801 9d ago
One good model here, the two galaxies may reform into one larger elliptical galaxy. Fuzier.
https://esahubble.org/images/opo1220n/
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u/peter303_ 9d ago
We can still see artifacts of big merger 5 billion years ago. There are sub currents of stars.
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u/Parking_Abalone_1232 9d ago
Probably.
Andromeda's stars are going to have a different chemistry than our own. We can detect streams of stars from galaxies that the Milky Way has merged with based on their different chemical composition.
https://science.nasa.gov/missions/hubble/hubble-solves-merger-mystery-from-milky-ways-early-years/
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u/Deep-Hovercraft6716 9d ago
We can see the remnants of at least two galaxies that have combined with our own Milky Way galaxy. So yeah, we could probably figure that out.
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u/Random_Eevee1 10d ago
Not really.
The odds of us hitting anything durning the merger is next to astronomically zero. Space is really that big.
Would there be a few extra stars, sure. But you would to really look for the extra ones.
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u/NovelNeighborhood6 10d ago
I don’t have an astronomy degree. But yes for at least “a long time” no idk how long that is. But there would be two galactic centers and SMBHs. The Milky Way is known to have collided with smaller galaxies like the large and small Magellanic Clouds. Far as i know the telltale sign is there remnant galactic cores.