Nearly Naked Dark Matter Halo

In two important papers (paper 1 and 2) put on astro-ph two days ago, a group of astronomers describe the observations and analysis of two colliding galaxy clusters which have temporarily stripped each other of their hot X-ray emitting gaseous cores respectively, leave nearly naked dark matter halos. The most important aspect of this set of papers is their confirmation of a major prediction of Cold Dark Matter theory which is not explainable by alternatively proposed solutions to the missing mass problem.

The holy grail for proving that cold dark matter halos exist, thereby also proving that dark matter exists, is being able to "observe" a naked halo. When baryons are present, there is always a way to modify gravitational theory, albeit rarely self consistently, or propose the existence of more cold (unobservable) baryons around a galaxy or cluster to explain observations of gravitational lensing and other phenomena, which are naturally explained by the presence of a dark matter halo. Because dark matter halos are so massive and most began forming very early in the universe, all of them have had a long enough time and have a large enough gravitational potential to pull in the intergalactic gas around them. Only some small halos are theorized to be truly "empty". And even if large naked halos existed, where would we look to find them? The next best observation would therefore be of a dark matter halo where the baryonic matter and the dark matter are clearly separated by a process that would leave no significant amount of cold baryons at the center of the halo. This is what this observation show.

Using the phenomenon of weak gravitational lensing, an effect well know to observers (easily seen by the Hubble telescope through the presence of the arced galaxies), one can determine the mass along the line of sight needed for such distortions in background galaxies. This method was used to determine the mass and center position of the two colliding dark matter halos. Because both of the galaxies clusters are at the same redshift and are traveling perpendicular to the plane of the sky away from each other they must have just made a pass through one another. The significance of this is that dark matter only interacts gravitationally allowing the halos to pass through each other. Baryons on the other hand collide, shock, and heat up which slows them down relative to the dark matter. The heated gas can be seen in the X-rays and is observed to have formed a bow shock behind the gravitational centers of the halos. It is impossible for much gas (baryonic matter) to have remained at the center of the halos and at the same time remained cold (no X-rays). Therefore, a modification of gravity exerted by baryons can not be used to explain this observation, nor can the presence of a large amount of cold unobservable baryons.

This is a great step in proving the existence of dark matter, but there is a long way to go to truly understanding the nature of dark matter and dark matter halos. It is, however, exactly this combination of weak and strong gravitational lensing observations combined with multi-wavelength observations that will lead the way. I will soon post an explanation of what these kinds of observations can tell us about the nature of dark matter.