<p>Galaxies grow through cosmic time by accretion of the surrounding matter. Some of its mass increase happens through smooth accretion of gas, but much also happens through merging with small clumps of dark matter, gas, and stars, called satellite galaxies. This is called "minor merging".</p>

<p>If merging galaxies are similar in size, it's called major merging. Most large galaxies have gone through of the order of one major merger through its life (<a href="http://arxiv.org/abs/1410.3479">Man et la. 2014</a>), but whether or not it happened for Andromeda, we cannot know for certain.</p>

<p>However, if you take a look at Andromeda, you'll see that it has quite a large <em>bulge</em>, i.e. the central, reddish part of the galaxy. This region is dominated by stars with a more "chaotic" velocity patterns. That is, their paths aren't "rotation-dominated" (lie in the disk), but more "dispersion-dominated" (leave the disk). To achieve such velocities, the system has to be disturbed by infalling matter, so a large bulge is probably a signature of a major merger. Investigating the velocity patterns of Andromeda, <a href="http://arxiv.org/abs/1502.03820">Dorman et al. (2015)</a> find that it has had a much more violent history than the Milky Way, and in fact models by <a href="http://arxiv.org/abs/1203.6081">Davidge et al. (2012)</a> and simulations by <a href="http://arxiv.org/abs/1010.0679">Hammer et al. (2010)</a> of the history of Andromeda suggest that <strong>8-9 billion years ago, Andromeda went through a major merger</strong>.</p>

<p>Note however that a large bulge <em>can</em> also be explained, at least in cosmological simulations, by a more smooth and "cold" accretion, i.e. gas that doesn't get shock-heated (Dekel et al.)</p>

CC BY-SA 3.0
2015-07-02T10:35:48.753Z
Note that a significant fraction of Andromeda's "bulge" may actually be the vertically thickened part of a  bar (a so-called boxy/peanut-shaped bulge), and thus not necessarily the result of a major merger; see [Athanassoula & Beaton (2006)](http://arxiv.org/abs/astro-ph/0605090).
Peter Erwin
CC BY-SA 3.0
2015-07-02T13:56:29.423Z
Thanks @Peter, good point. So apparently Andromeda has the worst possible inclination for us to study it…
pela
CC BY-SA 3.0
2015-07-02T15:47:16.257Z
Yeah, it's pretty annoying. If we could just tilt it about fifteen or twenty degrees....
Peter Erwin
CC BY-SA 3.0
2015-07-02T20:10:33.883Z
It would probably be easier to fly 200 kpc away from the Milky Way. But less challenging than tilting Andromeda!
pela
CC BY-SA 3.0
2015-11-05T07:32:24.097Z
We'll have a better look at it in 3 billion years or so.
userLTK
CC BY-SA 3.0
2015-06-25T09:13:58.930Z
2015-06-26T11:25:28.253Z
2015-06-26T11:25:28.253Z
pela
<p>We know that the Andromeda Galaxy is moving towards the Milky Way and vice versa, so there will be a collision. Because of the vast space between them, the  stars as they are defined will not themselves come into contact until much later if at any time at all.</p>

<p>My question is whether Andromeda has had a collision prior to this 4 billion year inevitability.</p>

<galaxy><galactic-dynamics><m31>
Has the great Andromeda Galaxy ever collided with any galaxies?
Isopycnal Oscillation
# Has the great Andromeda Galaxy ever collided with any galaxies?

(Asked by: username_0 on 2015-06-25)

We know that the Andromeda Galaxy is moving towards the Milky Way and vice versa, so there will be a collision. Because of the vast space between them, the stars as they are defined will not themselves come into contact until much later if at any time at all.

My question is whether Andromeda has had a collision prior to this 4 billion year inevitability.

**Tags:** galaxy, galactic-dynamics, m31

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## 1 answers

### Answer by username_1 on 2015-06-25. Score: 14 | Accepted answer

Galaxies grow through cosmic time by accretion of the surrounding matter. Some of its mass increase happens through smooth accretion of gas, but much also happens through merging with small clumps of dark matter, gas, and stars, called satellite galaxies. This is called "minor merging".

If merging galaxies are similar in size, it's called major merging. Most large galaxies have gone through of the order of one major merger through its life ([Man et la. 2014](http://arxiv.org/abs/1410.3479)), but whether or not it happened for Andromeda, we cannot know for certain.

However, if you take a look at Andromeda, you'll see that it has quite a large *bulge*, i.e. the central, reddish part of the galaxy. This region is dominated by stars with a more "chaotic" velocity patterns. That is, their paths aren't "rotation-dominated" (lie in the disk), but more "dispersion-dominated" (leave the disk). To achieve such velocities, the system has to be disturbed by infalling matter, so a large bulge is probably a signature of a major merger. Investigating the velocity patterns of Andromeda, [Dorman et al. (2015)](http://arxiv.org/abs/1502.03820) find that it has had a much more violent history than the Milky Way, and in fact models by [Davidge et al. (2012)](http://arxiv.org/abs/1203.6081) and simulations by [Hammer et al. (2010)](http://arxiv.org/abs/1010.0679) of the history of Andromeda suggest that **8-9 billion years ago, Andromeda went through a major merger**.

Note however that a large bulge *can* also be explained, at least in cosmological simulations, by a more smooth and "cold" accretion, i.e. gas that doesn't get shock-heated (Dekel et al.)

> **Comments:**
> **username_2**: Note that a significant fraction of Andromeda's "bulge" may actually be the vertically thickened part of a  bar (a so-called boxy/peanut-shaped bulge), and thus not necessarily the result of a major merger; see [Athanassoula & Beaton (2006)](http://arxiv.org/abs/astro-ph/0605090).  
> **username_1**: Thanks @Peter, good point. So apparently Andromeda has the worst possible inclination for us to study it…  
> **username_2**: Yeah, it's pretty annoying. If we could just tilt it about fifteen or twenty degrees....  
> **username_1**: It would probably be easier to fly 200 kpc away from the Milky Way. But less challenging than tilting Andromeda!  
> **username_3**: We'll have a better look at it in 3 billion years or so.  

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