# How Much Overlap Will the Andromeda Galaxy and the Milky Way Have When They Collide?

Measurements of Andromeda's blue shift let us conclude that the distance between the Andromeda galaxy and the Milky Way is decreasing and in a few billion years they will "collide".

The blue shift only yields the radial component of Andromeda's velocity vector. It is my understanding that measuring the tangential component is crucial in determining whether a "collision" will actually happen (in a gravitationally bound two body system, for point-like bodies to collide, the relative velocity component must point exactly towards the other body, i.e. the tangential component must vanish).

Now, galaxies are not point-like, so some small nonzero tangential component might lead to a collision where at least some galaxy arms intersect.

Has the tangential velocity been measured? If so, how? How central is the collision (bulge into bulge, bulge into arms, arms into arms)?

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Information on the tangential velocity of Andromeda is very easy to find. – Rob Jeffries May 31 '15 at 22:57
See also this question (which I posted to the old Astronomy beta site before it was merged into Physics). – Keith Thompson Jun 1 '15 at 1:28

The simulations show that the Milky Way and the Andromeda galaxy will merge, consistently with the radial (head-on collision) orbit deduced in Paper II for M31, and that the first “pericenter” (closest approach) will occur at around 4 Gyr from now. For our Milky Way, the encounter has 72.2% probability of being prograde (the galaxy spins in the same direction as the flyby). In 41.0% of the Monte-Carlo orbits M31 makes a direct hit with our Milky Way, where the authors define a “direct hit” as an encounter with a first pericenter distance less than 25 kpc. The two galaxies will eventually merge after 5.9 Gyr, and the radial mass profile of the merger remnant will be significantly more extended than the original individual profiles. Roughly speaking, this profile will follow the $R^{1/4}$ law characteristic of elliptical galaxies, in agreement with the predictions from the numerical simulations of major mergers of spiral galaxies.
I'll note that $25 \text{ kpc} = 25,000 \text{ pc} \cong 81,540 \text{ ly}$, and that the Milky Way is only about 100,000 ly in diameter, so the "direct hit" definition is quite a substantial distance here.