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I'm looking for something to cite about galaxy mergers in clusters of galaxies.

I need numbers on the frequency of mergers in the outer parts of galaxy clusters vs. the cluster center. In the center number densities are higher but so is the velocity dispersion.

Are mergers of galaxies more likely to occur in the outskirts or the central region of galaxy cluster?

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  • $\begingroup$ Do you have a preference on simulation vs. observational based evidence of galaxy merger rates? $\endgroup$
    – Aaron
    Aug 27, 2014 at 15:45
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    $\begingroup$ +1 good question. I cannot provide an answer, but in general merging in clusters is rather rare (because the galaxies' mutual velocity is $\sim1000$km/s, much higher than their own velocity dispersion), in contrast to galaxy groups. On the other hand, the central cluster galaxy is almost certainly a merger remnant, but may have merged before the cluster was established. Minor merging activity onto the central cluster galaxy is presumably common. $\endgroup$
    – Walter
    Aug 27, 2014 at 15:48
  • $\begingroup$ Well clusters form hierarchical, and galaxy do fall in in pairs and small groups so the velocity dispersion is not really an argument. Mergers are more frequent than one might think (Mihos 03, Gnedin 03, McGee 09, Berrier 09, DeLucia 12), but still by no means a common event. All the papers mentioned have touched on the subject one way or another but not quiet what I was looking for. $\endgroup$
    – con-f-use
    Aug 28, 2014 at 12:03
  • $\begingroup$ @Aaron: Both are interesting. $\endgroup$
    – con-f-use
    Aug 28, 2014 at 12:31
  • $\begingroup$ Mihos 03: ned.ipac.caltech.edu/level5/March07/Mihos/frames.html $\endgroup$
    – Wayfaring Stranger
    Aug 30, 2014 at 19:43

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Galaxy mergers in clusters explain the large central galaxies in clusters. In fact, the centers of clusters often host a special type of galaxy called a cD which is usually extremely massive because of galactic cannibalism. Dynamical friction makes galaxies spiral down toward the center where they get eaten by the central galaxy and that in turn makes the central galaxy more massive and more efficient at this process.

In the rest of the cluster, the velocity dispersion is significantly higher than the escape velocities from individual galaxies. So a) galaxies cannot gravitationally attract other galaxies because they go by too fast and b) even if there is a head on collision by sheer coincidence, they would pass through and not capture one another. There may well be huge consequences to both galaxies, such as stellar rings and shells formed and tidal tails extracted, but the two remnants would separate and perhaps never meet again.

This is an oversimplification though because clusters often have subgroups of galaxies which have lower velocity dispersion wrt each other and these could merge much as galaxies in groups merge. But the tidal field of the cluster actually works against this and so the rate would be lower.

There is a lot of literature on cD or central galaxies since it is one of the standard types of galaxies. Here is a recent one:

"Growth of brightest cluster galaxies via mergers since z=1", Claire Burke and Chris A. Collins, 2014, MNRAS, v. 434, p. 2856. http://mnras.oxfordjournals.org/content/434/4/2856

A study of merger rates in the outer parts of clusters may take a major effort of going through the results of N-body simulations on your own and your answer may depend on which simulation you use.

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