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Titanic merger of galaxy clusters revealed

  1. Sep 24, 2004 #1
    This is amazing, I haven't heard of any merger of two galaxy clusters until now.


    "One of the hottest, most energetic mergers of two colossal galaxy clusters has been imaged in exquisite detail by an X-ray observatory in space, astronomers announced on Thursday. The collision is one of the most powerful cosmic events ever witnessed by astronomers."

    It makes me wonder if there's some possibility that the Local group and the Virgo Cluster can merge in the future
  2. jcsd
  3. Sep 25, 2004 #2
  4. Sep 25, 2004 #3
    thanks Pelastration. The ESA link has answered my question:
    "Our Milky Way galaxy is part of a small group of galaxies but is not gravitationally bound to the closest cluster, the Virgo Cluster. We are destined for a collision in a few thousand million years, though. "

    and the animation of the two clusters colliding is indeed spectacular
  5. Sep 25, 2004 #4


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    This is indeed spectacular. For me it is surprising that the simulation shows the generation of two tidal tails (upper and lower), as in case of the collision between two galaxies with similar masses (the "antennae" galaxies are a beautiful example of this phenomenon). In this case this gas or debris tails seam to be larger than the cluster itself. I wonder also how much galaxy collisions are expected to take place during such an event.
    Last edited: Sep 25, 2004
  6. Sep 26, 2004 #5


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    Frankly, i do not understand what the excitement is all about. Although it is very spectacular, it is known a long time that galaxies merge and collide. The difference is that they now see more details, which is very nice. But it is presented like it is the discovery of the century.
  7. Sep 26, 2004 #6


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    To me the collision of galaxy clusters was unknown until now. I searched a little bit in internet and I found a paper with an overview of the subject (and detailed simulation methods):


    Surprisingly (at least to me), this events should be relatively frequent in universe models with hierarchical large-scale structure formation, although clusters are recent structures.
    Last edited: Sep 26, 2004
  8. Sep 26, 2004 #7
    this cluster of galaxies called MS1054-03 is the most distant that I know. It's eight billion light years away. Nice photo

    hellfire's link says that collisions between clusters are habitual. Can some member of PF identify signs of some merger in MS1054-03? We can do some detective research :cool:

    Can you imagine, if the cosmos is infinite, the quantity of mergers between galaxies clusters that are occurring right now?. Can this strange hierarchy of mergers continue to higher steps, I mean collisions between two superclusters, or even the higher step, collisions between two supercluster complexes? What strange universe we are living in!
    Last edited: Sep 26, 2004
  9. Sep 26, 2004 #8


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    I think there are clusters at z > 1 (a proper distance > 10 Gly). I would guess that such young clusters had no time for collisions.

    On the other hand, cluster collisions seam to take place along filaments or where filaments intersect. I belive that on greater scales than clusters or superclusters, such as the filaments, there will be no collisions because there is no matter flow as well as no rotation. I think this was already measured and supports the cosmological principle. In all LSS simulations I have seen the filaments remain static. They are formed but they do not move. (I am guessing so I may be wrong).

    It would be nice to know which are the observational "signs" of mergers of galaxy clusters. May be one can infer based on the geometry (e.g. tidal tails, shape, etc.) or may be based on x-ray emissions from the intracluster gas, which would be stronger than emissions in hydrostatic equilibrium...?
  10. Sep 27, 2004 #9
    hellfire, If you know the name of some of these clusters with z>1 I will be very grateful if you can post them. I like to investigate the properties of the objects in the frontier of our known universe

    I found a galaxy cluster that is still farther than MS1054+03. It's called CL1604+43 and has a redshift of z=0.9


    while MS1054+03 it's at a redshift of z=0.83, according to this paper
  11. Sep 27, 2004 #10


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  12. Sep 27, 2004 #11
    The cluster with higher redshift in the list is RXJ 0848.6+4453, with z=1.27
    If I go to Ned Wright calculator
    and using the standard parameters H=71, Omega lambda=0.73, Omega "baryon+dark matter"=0.27, flat universe, it gives a comoving radial distance of 12.8 billion light years. Wow!
  13. Sep 27, 2004 #12
    Well, I've not found galaxy clusters more distant, but I've found a proto-cluster with z=4.1! A proto-cluster is a galaxy cluster in phase of formation. This super-far proto-cluster discovered is called TNJ1338-1942

    Ned Wright's calculator yields a comoving radial distance of 24 billion light years. This means that we are seeing the cluster as it was when the universe had an age of 1'5 billion years

    Who must be living there now?
  14. Oct 16, 2004 #13
    This paper appeared Friday. They saw a great quantity of radio emitting galaxies in the cluster
    Abell 2125 II. They believe that the cluster is undergoing a merger with another cluster, and this is provoking an enhancement of the star formation rate, so the surplus of radio emitting galaxies is explained. Pretty interesting
    Last edited: Oct 16, 2004
  15. Dec 10, 2004 #14
    regarding the observational signs of a merging cluster;

    Generally you will see multiple peaks in x-ray images (where one peak is the main cluster core and the others are merging groups or sub-clusters). There may be "cold fronts" which are believed to be the remaining cool core from a merged sub-cluster or group (although this is not set in concrete, as some cold fronts are clearly not due to mergers). Keep in mind these "cold fronts" are only cold relative to the main cluster gas, they are still around 10 million degrees or so!

    In the optical, you may see multiple concentrations of galaxies. There will more than likely be a bi-modal velocity dispersion.

    All clusters that exhibit strong radio haloes are thought to be merging clusters (a radio halo is extended radio emission in a cluster, usually extending to about 1Mpc in diameter). However a merging cluster does not necessarily guarantee excess radio activity (due to agn or star formation) in the cluster galaxies.

    Regarding galaxy collisions in mergers;

    Not many! It is like when 2 galaxies merge, there are basically no collisions between the stars. Due to the size of a galaxy compared to the space between the galaxies in a cluster, not many galaxy-galaxy mergers occur. Also the velocity dispersion in clusters is large, so the probability of galaxies merging is low. However galaxies in cluster interact via tidal interactions, galaxy harassment and ram pressure stripping (interaction with cluster gas).

    Oh and you are not seeing tidal tails in the simulation, they are weak shock fronts propagating throught the intracluster cluster medium (the hot gas in permeating clusters). These are indeed a sign of a merger, however they are generally weak and not so prominant in x-ray images. Sound speedsd in the ICM are generally around 500km/s, infalling clusters generally have velocities of this order or a little more, hence the shock front.

    Clusters pretty much continuously acrete matter in the form of gas, galaxies, groups of galaxies and via cluster-cluster mergers. However the cluster-cluster mergers are generally less frequent.

    hope that helps! any more questions, feel free to ask!

  16. Dec 10, 2004 #15


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    Cool, thank you for this excelent post. I am still trying to understand properly what you wrote, but I have already some questions.

    You mentioned that cluster merges show radio haloes. Is this radio emission usually detected in the same volume as the one occupated by the intracluster gas emitting x-rays, or may be more outside, or in a bigger volume? I also would like to know how it follows that a radio halo arises when a merger takes place (what is the physicall explanation for this). I assume radio emissions are due to synchrotron radiation from charges in magnetic fields, so why does this phenomenon arise especially in case of mergers?

    Also regarding the number and dynamics of mergers: how far does this depend on the cosmological model? Does a universe without dark energy lead to significantly more merges that one with dark energy?
  17. Dec 10, 2004 #16
    The radio haloes are generally less extended than the x-ray emission.

    You are right, the emission is synchrotron radiation due to electrons spiraling in the cluster's magnetic field. The acceleration of these electrons to relativistic speeds is not known precisely, however 2 scenarios are commonly discussed. They are turbulent reacceleration and acceleration due to shock fronts produced by the merger. I think it may have been shown recently that turbulent reacceleration is the most likely candidate. I can't remember the exact physics of turbulent reacceleration, but if you give me some time I can remind myself of the details.

    not so sure about the last question! Maybe someone else can field this one!
  18. Dec 10, 2004 #17
    Thanks for the info, and please stay around a while at PF
    I found this paper in which is suggested that there is a merging of at least two subclusters in the galaxy cluster Abell 1367.


    For me, the idea of merging of subclusters is new. I suppose that the merging of two subclusters would not produce very much galaxy collisions, right?
  19. Dec 10, 2004 #18
    yes you are right, the galaxies in a cluster-cluster merger are collisionless, much like stars in a galaxy-galaxy merger. The gas however, is collisional, hence the x-ray properties are generally a definative way of telling if a cluster is interacting.

    If you go to adsabs.harvard.edu or arxiv.org and type in galaxy cluster, merger, infall etc. I am sure you will find many more papers!

  20. Dec 11, 2004 #19


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    Welcome to Physics Forums matt.o!

    Just to add a word about detection of cluster mergers/collisions in the optical: it's difficult to cleanly distinguish between superposition (one cluster behind the other) and physical interaction in the optical ... if you have only images (no redshifts), even more difficult. As is often the case, combining optical (images+redshifts), X-ray, and radio helps 'make the case' for a merger/collision. I'm not sure how much adding IR (IRAS, 2MASS, Spitzer), microwave (esp the SZE), and gamma images will improve things; adding 'redshifts' in other parts of the EM (e.g.a new X-ray technique) will likely be another powerful tool.
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