Hubble Spots Mysterious Dark Matter ‘Core’

Article written: 2 Mar , 2012
Updated: 23 Dec , 2015

[/caption]Astronomers are left scratching their heads over a new observation of a “clump” of dark matter apparently left behind after a massive merger between galaxy clusters. What is so puzzling about the discovery is that the dark matter collected into a “dark core” which held far fewer galaxies than expected. The implications of this discovery present challenges to current understandings of how dark matter influences galaxies and galaxy clusters.

Initially, the observations made in 2007 were dismissed as bad data. New data obtained by the Hubble Space Telescope in 2008 confirmed the previous observations of dark matter and galaxies parting ways. The new evidence is based on observations of a distant merging galaxy cluster named Abell 520. At this point, astronomers have a challenge ahead of them in order to explain why dark matter isn’t behaving as expected.

“This result is a puzzle,” said astronomer James Jee (University of California, Davis). “Dark matter is not behaving as predicted, and it’s not obviously clear what is going on. Theories of galaxy formation and dark matter must explain what we are seeing.”

Current theories on dark matter state that it may be a kind of gravitational “glue” that holds galaxies together. One of the other interesting properties of dark matter is that by all accounts, it’s not made of same stuff as people and planets, yet interacts “gravitationally” with normal matter. Current methods to study dark matter are to analyze galactic mergers, since galaxies will interact differently than their dark matter halos. The current theories are supported by visual observations of galaxy mergers in the Bullet Cluster, and have become a classic example of our current understanding of dark matter.

Studies of Abell 520 are causing astronomers to think twice about our current understanding of dark matter. Initial observations found dark matter and hot gas, but lacked luminous galaxies – which are normally detected in the same regions as dark matter concentrations. Attempting to make sense of the observations, the astronomers used Hubble’s Wide Field Planetary Camera 2 to map dark matter in the cluster using a gravitational lensing technique.

“Observations like those of Abell 520 are humbling in the sense that in spite of all the leaps and bounds in our understanding, every now and then, we are stopped cold,” said Arif Babul (University of Victoria, British Columbia).

Jee added, “We know of maybe six examples of high-speed galaxy cluster collisions where the dark matter has been mapped, but the Bullet Cluster and Abell 520 are the two that show the clearest evidence of recent mergers, and they are inconsistent with each other. No single theory explains the different behavior of dark matter in those two collisions. We need more examples.”

The team has worked on numerous possibilities for their findings, each with their own set of unanswered questions. One such possibility is that Abell 520 was a more complicated merger than the Bullet Cluster encounter. There may have been several galaxies merging in Abell 520 instead of the two responsible for the Bullet Cluster. Another possibility is that like well-cooked rice, dark matter may be sticky. When particles of ordinary matter collide, they lose energy and, as a result, slow down. It may be possible for some dark matter to interact with itself and remain behind after a collision between two galaxies.

Another possibility may be that there were more galaxies in the core, but were too dim for Hubble to detect. Being dimmer, the galaxies would have formed far fewer stars than other types of galaxies. The team plans to use their Hubble data to create computer simulations of the collision, in the hopes of obtaining vital clues in the efforts to better understand the unusual behavior of dark matter.

If you’d like to learn more about the Hubble Space Telescope, visit:


14 Responses

  1. lcrowell says

    This is an extreme conjecture, but maybe the two black holes of the galaxies collided. Dark matter with both galaxies was gravitationally bound to these black holes and some of it remained in the gravitational potential of the coalesced system.


    • magnus.nyborg says

      Maybe it is a pocket of almost only dark-matter that was at the center to begin with, and the surrounding galaxy clusters havent disturbed it much (yet)

      • Peristroika says

        That’s why they were drawn together in the first place. And in a billion years, they’ll all settle down together and raise some nice little star clusters.

      • magnus.nyborg says

        …I do not even want to contemplate how they procreate…

  2. JonHanford says

    “There may have been several galaxies merging in Abell 520 instead of the two responsible for the Bullet Cluster.”

    I think you meant to refer to subclusters of galaxies instead of individual galaxies merging. Otherwise a nice article on these new Hubble observations of Abell 520. The paper by Jee et al is available here:

    Also good that you note there are several possible scenarios that explain what is found without resorting to a modification of DM theory. Sec. 4 of the linked paper goes into detail concerning the possible alternatives.

    • IVAN3MAN_AT_LARGE says

      I can always count on you, Jon, to find the relevant paper whenever I don’t get around to doing so. Thanks!

      • JonHanford says

        Mutual appreciation for your superior paper finding abilities Ivan. You save me a lot of time! :^)

    • lcrowell says

      I don’t like the idea of DM being “sticky” or having a larger interaction with itself. This would be particularly troublesome if the DM neutralino is a Majorana fermion. Also if it is “sticky” here, then why not with the Bullet cluster? If dark matter is essentially conservative in its dynamics by interacting only gravitationally, then the DM must have been gravitationally bound to a large gravitating system which exhibited large dissipative interactions during this collision.


  3. The Muss says

    Pu the religion fanatic on dark energy, the 1st idea will be “HELL”

    I’d like to see that event

  4. Torbjörn Larsson says

    Ha, the missing ordinary matter to predict dark matter observations.

    And so it goes…

  5. Efreet says

    there is one other possibility implied which i will not bother to mention due to the fact that it will generate indignant responses from the peanut gallery. you know which one i’m referring to…

  6. I wonder if this could be an example of something I think is quite possible, which is that there could a nearby universe bound inside of a three-brane that is close enough to our three-brane universe such that the matter in that other universe is gravitationally affecting matter in our universe, or I should say affecting the shape of spacetime in our universe.

    This is one of the theories that is not prevented by any observations so far and supported by string theory. I have read about several examples of dark matter that is believed to be bound to a galaxy, or better yet, where a galaxy is centered around a halo of dark matter, but where the two are kind of the same object. Or at least the visible and dark matter components are bound together by gravity.

    We have seen galaxies that have collided and where the visible matter in the two galaxies has slowed down during the collision but where the dark matter has kept on going. In these cases, it seems that the dark and visible matter can get pulled apart because the visible matter interacts while the dark matter just passes through.

    But what about this case? There seems to be a source of gravitational pull there which is assumed to be dark matter in our universe, but could this actually be matter such as a galaxy in another nearby three-brane to which our only connection is through gravity?

    Imagine this. There is a three-brane universe, say one astronimical unit away from ours in a dimension in which our universe cannot move, and that universe contains a star like the sun. Wouldn’t matter in our universe clump around this area in our universe where it feels the gravity of the star that is just one AU away? Or maybe it is just an inch away!

    I keep looking for examples like this hoping it will provide evidence for this theory, which I got mainly by reading Brian Greene.

    • what? no replies? what do smarter people than me think about “my theory” about the dark matter that really seems not to have a corresponding galaxy associated with it? By “my theory” I mean where I think there could be a nearby universe gravitationally affecting ours that I read about being a possibility in Brian Greene’s books.

  7. Astrofiend says

    As I read in a nice summary paper the other day, we are only just starting to understand DM. At the moment we describe the Baryonic universe with hundreds of different equations and constants, with exquisite and subtle physics the norm. We currently describe DM with a few measly parameters and bugger all else (that is concrete). It would be foolish to think that DM physics can be described so simply, despite it’s overwhelming success in describing observations and it’s keystone status in cosmological theory. DM may well prove to show very rich physics, which we are only at the very beginning of understanding. It’s an exciting time to be working on such problems…

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