RWTH Aachen Particle Physics Theory

Dark matter mysteries

July 20th, 2014 | by

The bullet cluster are two galaxy clusters roughly 3.8 billion light years away in the Carina constellation in the southern sky.  Galaxy clusters are gravitationally bound accumulations of galaxies. The bullet cluster is an object of particular interest: Since its discovery in 1995, it has been an object of study with different observation methods. In the optical light, there seem to be two separate galaxy clusters with a distance of roughly 0.7 Mpc. The X-ray observation reveals, that these two galaxy cluster collided in the past and are now separating again. The bullet cluster is a textbook example for such two objects interacting, leading to a bow shock which can be nicely studied in the X-ray image of the object. However, there is something else which is very interesting about this object: The collision separates two components of the galaxy clusters, namely the luminous mass of the cluster and the main mass components of the cluster, that can not be seen in the optical or X-ray region. This hints towards a large amount of dark matter taking part in the collision. And this makes it very interesting for particle physicists as well!

Let’s look at what happened in these clusters. Two galaxy clusters approach each other and collide:


You can see here three different colored components of the clusters: yellow being the galaxies, pink is the intergalactic gas, and blue indicating the concentration of mass of the clusters. Every galaxy cluster comes with a lot of gas between the galaxies.



The gas mainly consists of ionised hydrogen and helium. Important to know here is that the mass of this gas outnumbers the mass of the galaxies by a factor of 5-10, so that the most visible mass is the gas and not the galaxies (hard to believe, right?).



Since the galaxies in this cluster are just too sparsely distributed, they barely interact at all and just pass through the other cluster. What actually interacts, is the gas of both galaxies. Basically the two different sized gas fronts collide, and both slow down.



You can find a nice animation of the event here. So with time, the galaxies of the two clusters separate again. The gas fronts have lost velocity and lag behind the galaxies. The smaller gas front  leaves a trace in the larger one, which resembles a bullet shot in a medium, hence the name, bullet cluster.



bcThis is the scenario one observes today, where I tried to adopt the color code from.
In yellow you can see the luminous galaxies of the two clusters (Hubble image,the galaxies makes up ~1-2 % of the clusters’ mass), pink is the gas (measured with X-ray images of Chandra, 5-15%), and blue is the region that contains most of the mass of these clusters. The latter distribution was determined via gravitational lensing effects (see e.g. here and here).
So the interesting effect here is that the part that makes most of the mass in the visible light (the gas, red), is separated from the region, where large amounts of (non-luminous) masses have to be. These were determined with gravitational lensing (in blue). This implies, that the stuff that makes up most of the galaxies mass are rarely interacting with each other or with the galaxies/gas and just crossed the other cluster (together with the visible galaxies).

abellA similar observation is the “train wreck cluster” Abell 520 which is a galaxy cluster in the Orion constellation. Here, the collision or probably collissions are not yet clear from observation. The matter components, that are not luminous, are somehow stuck in the center, while the galaxies have passed through each other as in the bullet cluster example. That seems to contradict the bullet cluster example of separation of gas and massive parts of the cluster. Possible explanations are discussed in the literature (see e.g. here) but no final conclusion could be drawn so far. The are two main explanation direction: either, there are multiple collisions of  galaxy clusters that make the whole picture very complicated, or there is a new component of interaction of the non-luminous matter that makes it stay in the center of the collision. Maybe the second option gives us some more hints towards what the dark matter could be and how it interacts with itself and with ordinary matter. For example, from understanding these collisions, it is possible to give upper limits on the self-interaction of the dark matter.

For some musical interpretation see here.

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