Andromeda Galaxy Eating the Neighborhood

by Nancy Atkinson on September 3, 2009

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An artist's rendering shows the spiral galaxy of Andromeda, center right, over a period of about three billion years as repeated, but modified views of the dwarf galaxy Triangulum, move away from it, clockwise towards Earth, then back towards it, where Triangulum will be ultimately devoured by the Andromeda galaxy says astronomer John Dubinski. (AP Photo/Illustration by John Dubinski and Larry Widrow)

An artist's rendering shows the spiral galaxy of Andromeda, center right, over a period of about three billion years as repeated, but modified views of the dwarf galaxy Triangulum, move away from it, clockwise towards Earth, then back towards it, where Triangulum will be ultimately devoured by the Andromeda galaxy says astronomer John Dubinski. (AP Photo/Illustration by John Dubinski and Larry Widrow)


From Earth, the Andromeda Galaxy looks like a calm, bright galaxy, and is visible with the naked eye in our night sky. But astronomers have discovered things aren’t as tranquil as it seems over at M31. Andromeda is eating the neighbors.

The Andromeda Galaxy contains a trillion stars and lies only about 2.5 million light-years away, so it is a great object to observe and study. But recently astronomers observed wispy streams of stars on the outer fringes of Andromeda, and realized they were leftovers from a cannibalistic feeding frenzy of smaller galaxies it has absorbed.

“This is a startling visual demonstration of the truly vast scale of galaxies,” said Dr. Mike Irwin from the University of Cambridge. “The survey has produced an unrivalled panorama of galaxy structure which reveals that galaxies are the result of an ongoing process of accretion and interaction with their neighbours.”

The cannibalism continues and another victim lies in wait: M33 in the constellation of Triangulum, is destined for a future meal.

“Ultimately, these two galaxies may end up merging completely,” Dr. Scott also from the University of Cambridge. “Ironically, galaxy formation and galaxy destruction seem to go hand in hand.”

Astronomers from Cambridge were part of an international team that made a million light-year-wide survey of the Andromeda Galaxy and its surroundings using a powerful digital camera on the giant Canada-France-Hawaii telescope on Mauna Kea, Hawaii.

They discovered that many of these stars could not have formed within Andromeda itself because the density of gas so far from the galaxy’s core would have been too low to allow formation to take place. Therefore, the team reason that they are almost certainly the remnants of other, smaller galaxies which have been absorbed by Andromeda – and that Andromeda itself is still in a state of expansion.

The team’s paper argues that the larger-scale substructures identified on the galaxy’s fringes are probably the “undigested” remains of previously accreted dwarf galaxies. In all likelihood, they originally belonged to dwarf galaxies or other, proto-galactic fragments.

Article in Nature.

Source: PhysOrg

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Nancy Atkinson is Universe Today's Senior Editor. She also is the host of the NASA Lunar Science Institute podcast and works with Astronomy Cast. Nancy is also a NASA/JPL Solar System Ambassador.

ILOVETHESTAR September 3, 2009 at 8:54 AM

Simply put, BIG fish eat little fish.

idlelimey September 3, 2009 at 9:03 AM

I’m sure I have read that M31 is on a collision course with the Milky Way too. Can’t we all just get along?

Jon Hanford September 3, 2009 at 9:27 AM

Sorry to repost, but “Over the past few years, researchers have been looking at the possibility of a past encounter between the Andromeda Galaxy and M 33 and a possible future merger. A new paper and movie that illustrates the orbital dynamics between M 31 and M 33 can be found here: http://arxiv.org/abs/0909.0398 . M33 is, of course, the third largest galaxy in the Local Group.”

Jon Hanford September 3, 2009 at 9:28 AM

A 2008 paper outlined a detailed study of an ‘HI bridge’ between M 31 and M 33. An e-print of the paper here: http://arxiv.org/abs/0807.1161 . Add to this the gradual infall of the Local Group into the Virgo Cluster, the distant future look of our local universe will appear quite different.

Torbjorn Larsson OM September 3, 2009 at 4:44 PM

Lawrence, as dark matter plays the dominant role, I’m even more at a loss. As UT just mentioned, DM stabilizes galaxy mergers. For mostly non-interacting DM I naively see mostly the virial theorem (by way of gravity bounding the overall system) and radiation as the mechanisms that glob the systems together.

Or are there other mechanisms that can enhance frictional losses, such as something like the “churning” in spirals (mixing different metallicity stars in the disc) I just heard about?

From Alkalurops September 3, 2009 at 9:56 AM

I have been asking the same question and looked everywhere for the paper in arxiv.org but I don’t seem to have the right keyword to match this study. Then I went to another forum and they did tell me there is such a study showing M33 would most most likely be devoured by M31. At least it finally appeared here in UT and showed the timescale which will most likely coincide with our galaxy colliding with M31 also. I wonder if our collision with it is taken into account. That should matter, right? Since MW is either as massive or more massive than M31 that should be considered as another variable in the simulation. That would be fun. It would be cooler if M32 survives all those 3 billion years. It would be punching holes through 3 highly distorted spirals.

ILOVETHESTAR September 3, 2009 at 10:09 AM

Thank you for the links to the papers. I did not realize M33 most likely had an encounter with M31. I now imagine if M33 has a 2nd encounter with M31, the results after the ‘streaming bridges’ has finished being visible for all intents and purpose, M33 will lock like M82 . M33 having a 3rd encounter with M31 will result in M33 being no more!!! M31 is just too big and gravitationally massive for M33 to mess with , although M33 is still a good size galaxy but smaller than our Milky Way galaxy

Lawrence B. Crowell September 3, 2009 at 2:56 PM

I have to bear a chest of ignorance here. I see these numerical simulations of galaxies colliding and of course the images of the antenna galaxy and the like in collision. A galaxy when averaged over the entire volume is largely a vacuum. There are some compact objects, such as stars and the like, with a very diffuse gas filling most space. I should think that a collision of two galaxies would largely see the stars of the two galaxies pass by each other relatively uneffected, but for some gravitational scattering. In these simulations there seems to be some sort of friction or drag which sticks the two galaxies together so they eventually coalesce into a large elliptical like galaxy.

Newtonian mechanics is conservative and I would expect the motion of stars to carry on their merry way. Is most of the stuff which sticks together largely just gas? If the merged galaxy is so formed then if later it has stars I would expect these stars to come from star burst (birth) activity. So a large galaxy formed from a collosal merger should then have a swarm of stars or their burned out cores (dwarfs, neutron stars and black holes) orbiting around it.

Lawrence B. Crowell

William928 September 3, 2009 at 5:04 PM

Since the universe, and by extension galaxies are continuously expanding, wouldn’t the absorption of smaller galaxies by those with much greater mass be relatively commonplace?

William928 September 3, 2009 at 5:44 PM

Now that I think about it, as the Universe expands, galaxies are actually moving away from one another. As previous posts have indicated, the devouring of those that are smaller by those that are larger is simply a function of their relative size and mass, as well as the role of dark matter, as Torbjorn Larsson, OM points out. In any event, it is another fascinating aspect of our universe.

Lawrence B. Crowell September 3, 2009 at 7:03 PM

The Bullet galaxy is a case where the DM components of the two galaxies in collision are seen to move forwards by Einstein lensing, while the luminous matter sort of clumps together. This has been the clinching evidence for DM.

I can well enough see how two colliding gas clouds will clump, as statistically the chance of molecules colliding is good. But it seems that stars would carry one without much disturbance.

The galaxies colliding and coalescing into larger ones exist in galaxy clusters. These are gravitationally bound systems. Really galaxy clusters are moving apart by the Hubble relation v = Hd.

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