HOWDY, NEIGHBOR: The Andromeda Galaxy, which may someday collide with our own Milky Way Galaxy, appears to have a long history of devouring smaller companions.
Image: Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSF
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A highly detailed survey of the nearby Andromeda Galaxy, which as a nearby spiral galaxy is something of an observational proxy for our own Milky Way, shows the remnants of smaller galaxies that our neighbor appears to have cannibalized.
The Pan-Andromeda Archaeological Survey is not yet complete but is already producing scientific results such as those in this week's Nature. (Scientific American is part of the Nature Publishing Group.) The study's high-resolution look at Andromeda provides further confirmation of the prevailing theory of galactic growth—that today's giant galaxies fed on smaller companions to reach their present size.
The survey, which relies on observations made at the Canada–France–Hawaii Telescope atop Mauna Kea in Hawaii, is led by astrophysicist Alan McConnachie, a research fellow at the National Research Council Canada's Herzberg Institute of Astrophysics in Victoria, British Columbia.
Astronomers enjoy studying Andromeda in part because of its proximity—at 2.5 million light-years away it is close enough to us that individual stars can be resolved within the galaxy. And Andromeda can be seen in its entirety, unlike the Milky Way, in which we are deeply embedded. "If you're inside a city, and you're trying to figure out what that city looks like, it's much harder," McConnachie says. What is more, he adds, Andromeda appears to be a fairly representative spiral galaxy, based on comparisons with others in the universe at large, so unraveling the history of its formation may help astronomers understand how galactic processes work in general.
The dominant cosmological theory holds that galaxies grow through a process of accretion—by cannibalizing smaller galaxies and incorporating their mass in an ever-growing amalgamation. In agreement with similar observations of the Milky Way, the Andromeda survey has found a sort of archaeological record of this process—the fossilized remains of devoured galaxies.
"We have this idea of how we think galaxies form, and that's to do with the merging together of smaller galaxies," McConnachie says. "And if that's true, then when we look over the vast area around a galaxy, we should actually be able to see remnants of the formation process." Those remnants, which McConnachie calls "the partially digested remains of these dwarf galaxies," take the form of large, diffuse streams of stars, former galactic groupings that have been pulled apart by the larger galaxy's gravitational pull.
The Andromeda survey is turning up several of those remnants, just as predicted by the accretion theory. "When I look at this picture, it really makes me happy," says James Bullock, a cosmologist at the University of California, Irvine, who has worked on what galactic formation residues should look like. "This is exactly the kind of thing that we would expect—these streams of stars, this pretty clear evidence that you've had all these past accretion events."
Bullock notes that this is not the first time such evidence has been found—in fact, some of McConnachie's co-authors published evidence for galactic cannibalism in Andromeda in 2001. But each increasingly detailed look at the galaxy provides more examples and, hence, a more convincing case. "You're never that confident in your theory until you really see it proved by observation," Bullock says. "So it's not humdrum; it's exciting in that sense."
The new survey also indicates that Andromeda's growth continues apace, and on a massive scale to boot. The panoramic portrait reveals a possible past interaction between the galaxy and a smaller neighbor, Triangulum, as evidenced by a newly discovered stellar appendage on Triangulum that extends in Andromeda's direction. That feature, based on the research team's simulations, could be explained by a close encounter between the two galaxies in the past few billion years that ripped a section of stars from Triangulum.
"It actually looks as if it is being destroyed by Andromeda," McConnachie says, which would be a surprise given how much distance separates the galaxies. If this assessment is correct, McConnachie says, then in a few billion years' time—around the same time Andromeda and the Milky Way are expected to come together—Andromeda will finish off Triangulum, creating an even larger supergalaxy.
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13 Comments
Add CommentFascinating.
Reply | Report Abuse | Link to thisSince the observed light from Andromeda is 2.5 million light years away, my question is whether the galactic cannibalism is actually taking place at this very moment, or whether this is something that took place 2.5 million years ago, and that if we were to observe the light emitted from Andromeda circa 2009, the merger would already have been completed?
A confused layman.
It took place but the present result is 2.5 million light years away as viewed from the Earth.
Reply | Report Abuse | Link to thisIf you could get to Andromeda using a worm hole or some other time travel method you could see its current state of cannibalism before the rest of us land bound humans.
Don't forget to take lots of pictures if you go, and come back twice as fast as you left so everyone you know will still be alive
here on Earth
As stated in the article the cannibalism will that place in a few billion years. A few million years is nothing on this time scale. So the situation would look very much like it does from our vantage point. Some shift in relative position, but I don't even know if Triangulum would be closer or farther away right now. The trailing of stars suggest it is still receding from Andromeda, but it could be falling inward in a long arc. The article didn't go into the details of the prediction of collision in a few billion years. Maybe they don't know exactly what the dynamics are but know it will eventually fall in.
Reply | Report Abuse | Link to thisFrom now on Andromeda will a pejorative for the accretion of influence in government, business, and politics.
Reply | Report Abuse | Link to thisVote Andromeda!
Just curious, but can any interpretation offer clues to dark matter accretion? It seems to me that if dark matter is assumed to reside in the galactic halo there should be some evidence of it, no? Most curious either way...
Reply | Report Abuse | Link to thisIn simulations of galactic encounters the streamers go in both directions. There should be observational evidence on whether these two are approaching or not. The article implies that they are gravitationally bound.
Reply | Report Abuse | Link to thisThat leads to my question. Granted the MW and A are going to collide, but will the remnants be gravitationally bound?
What happens to living entities when their galaxy (and therefore) their solar system is "gobbled up" by Andromeda?
Reply | Report Abuse | Link to thisTerry Thomas
Atlanta, Georgia USA
I know that the author John Matson has truly conveyed to me that the subject matter about the Andromeda's Galaxy is all about. I enjoyed this article.
Reply | Report Abuse | Link to thisI understood the article as to where it was clarified in much detail as you were able to read and click on the links throughout the article and enjoy reading the sub article and its connection with the Andromeda Galaxy. I enjoyed reading about how the gravitational pull is forming the galaxy to become larger. This is very interesting to me. It has my eyes and ears open.
I feel that the article is written so that the average person can understand. I don't think it can get any clearer.
I believe the information in the article because, once again you have the links you could click on to check out the sub-links and read about the Thirty Meter Telescope in Mauna Kea. Also, click on "Scientific results such as those in this week's Nature". I feel priveleged to be in this class and being able to learn about the Solar System.
I'm enjoying myself. Sondra Johnson
I know that the author John Matson has truly conveyed to me that the subject matter about the Andromeda's Galaxy is all about. I enjoyed this article.
Reply | Report Abuse | Link to thisI understood the article as to where it was clarified in much detail as you were able to read and click on the links throughout the article and enjoy reading the sub article and its connection with the Andromeda Galaxy. I enjoyed reading about how the gravitational pull is forming the galaxy to become larger. This is very interesting to me. It has my eyes and ears open.
I feel that the article is written so that the average person can understand. I don't think it can get any clearer.
I believe the information in the article because, once again you have the links you could click on to check out the sub-links and read about the Thirty Meter Telescope in Mauna Kea. Also, click on "Scientific results such as those in this week's Nature". I feel priveleged to be in this class and being able to learn about the Solar System.
I'm enjoying myself. Sondra Johnson
Perhaps the most important issue we or the people of future (a few million years later) shall be concerned about is how our Solar System will be involved in this cannibalism process taken place between Andromeda and milky Way. Will our Sun leave us and move away, pulled by the huge gravity of our neighbor galaxy? If our Sun is pulled away from us, will that take us with her? Somewhere else I read that our Solar System may not be merged with Andromeda, simply because it is placed in a very calm part of one of Milk Way's calmest arms. If so, what will be the fate of our home?
Reply | Report Abuse | Link to thisWhy, as I said in the Survey, is there no scale on the Andromeda photograph? It is familiar enough to not really need it in this case, but why no scale on *any* astronomical photograph?
Reply | Report Abuse | Link to thisThis article in my opinion was really interesting and believe that the author John Matson did a really good job at conveing his his point to this subject. I was really amused by how theirs a theory about how the two galaxies Andromeda and Triangulum will eventually come together.
Reply | Report Abuse | Link to thisThis article was a little more challenging than the past articles although it was very interesting and kept me wanting to finish it. The average reader could most likely understand this article the only hard thing about it is the vocabulary but besides that it was good.
The information on this article was very convincing exept that its all based on theorys. But I was most convinced that the two galawies will eventually come together.
It's not necessary to discuss why galaxy is falling apart.
Reply | Report Abuse | Link to thisThe whole new universe has come.