When there is a collision of 2 disc shaped galaxies, there is a tail formation created from both the galaxies. I read here that this was due to tidal forces, but I couldn't figure out how this happens. Is there a simple explanation as to why and how tidal tails are formed?

Also, are tidal tails in the same plane as the spin of the disc galaxy?

Here's an image showing the tails

  • $\begingroup$ There is an xscreensaver module that computes these collisions in a highly simplified model (COM plus first order corrections) and it gets a version of this behavior, so it's origin has to be fairly near the surface. $\endgroup$ Mar 9, 2015 at 1:30
  • $\begingroup$ @dmckee Sorry, but I have no idea what you mean by that. I simulated it with a 3-body code and it works and I could see the tails. My question is mainly on the theory of it. Why would it act so? How is the tidal force creating such a shape? $\endgroup$ Mar 9, 2015 at 5:53

1 Answer 1


The best paper on I've found on tidal tails is Reshetnikov & Sotnikova (2000).

Their simple description of tidal tail formation is:

To understand the development of tidal tails, one must recall how the water surface of the oceans get stretched radially by differential gravitational attraction exerted on it by our Moon. The differential forces between near and far side of the Earth depend on the third power of the Moon’s distance from Earth. That is why the oceans tides are rather mild. When two galaxies experience a close encounter with perigalactic distance is compared with galaxies’ sizes, the tidal field of one galaxy stretches its neighbour radially and then the galaxies’ rotation shears off stars and gaseous clouds from outskirts of their parent galaxies. As a result, stars on the far side of each disk is ejected into long and thin tails.

You wrote

Also, are tidal tails in the same plane as the spin of the disc galaxy?

Not always. An example is the globular cluster Messier 12, which orbits the Milky Way outside of the galactic plane. It's not really dramatic, though. This type of interaction is common in globular clusters; Wikipedia lists Palomar 5 as an example.

The thing is, Palomar 5 could also be classified as a stellar stream. You could argue that the terminology difference is just semantics, but I don't know what astronomers prefer. A more dramatic example is the Sagittarius Stream, part of the Sagittarius Dwarf Spheroidal Galaxy, which orbits the Milky Way.

You may find this paper interesting.

Some images of globular clusters and stellar streams:

The Sagittarius Stream:

(source: nova.org)

The Young Blue Stellar Stream in HGC 5128 (as investigated here):

A stellar stream in NGC 4651:

Some of these may be tidal tails, though that could be disputed, as they come from dwarf satellite galaxies which may be entirely consumed in the process.

  • $\begingroup$ The link to the picture of The Young Blue Stellar Stream in HGC 5128 seems dead now, can you update it? $\endgroup$
    – Urb
    Mar 17, 2021 at 17:13

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