Heart of Darkness

Now here’s a funny thing. I’ve been struggling to keep up with matters astronomical recently owing to pressure of other things, but I could resist a quick post today about an interesting object, a galaxy cluster called Abell 520. New observations of this complex system – which appears to involve a collision between two smaller clusters, hence its nickname “The Train Wreck Cluster” – have led to a flurry of interest all over the internet, because the dark matter in the cluster isn’t behaving entirely as expected. Here is the abstract of the paper (by Jee et al., now published in the Astrophysical Journal):

We present a Hubble Space Telescope/Wide Field Planetary Camera 2 weak-lensing study of A520, where a previous analysis of ground-based data suggested the presence of a dark mass concentration. We map the complex mass structure in much greater detail leveraging more than a factor of three increase in the number density of source galaxies available for lensing analysis. The “dark core” that is coincident with the X-ray gas peak, but not with any stellar luminosity peak is now detected with more than 10 sigma significance. The ~1.5 Mpc filamentary structure elongated in the NE-SW direction is also clearly visible. Taken at face value, the comparison among the centroids of dark matter, intracluster medium, and galaxy luminosity is at odds with what has been observed in other merging clusters with a similar geometric configuration. To date, the most remarkable counter-example might be the Bullet Cluster, which shows a distinct bow-shock feature as in A520, but no significant weak-lensing mass concentration around the X-ray gas. With the most up-to-date data, we consider several possible explanations that might lead to the detection of this peculiar feature in A520. However, we conclude that none of these scenarios can be singled out yet as the definite explanation for this puzzle.

Here’s a pretty picture in which the dark matter distribution (inferred from gravitational lensing measurements) is depicted by the bluey-green colours and which seems to be more concentrated in the middle of the picture than the galaxies, although the whole thing is clearly in a rather disturbed state:

Credit: NASA, ESA, CFHT, CXO, M.J. Jee (University of California, Davis), and A. Mahdavi (San Francisco State University)

The three main components of a galaxy cluster are: (i) its member galaxies; (ii) an extended distribution of hot X-ray emitting gas and (iii) a dark matter halo. In a nutshell, the main finding of this study is that the dark matter seems to be stuck in the middle of the cluster with the X-ray gas, while theĀ  visible galaxies seem to be sloshing about all over the place.

No doubt there will be people jumping to the conclusion that this cluster proves that the theory of dark matter is all wrong, but I think that it simply demonstrates that this is a complicated object and we don’t really understand what’s going on. The paper gives a long list of possible explanations, but there’s no way of knowing at the moment which (if any) is correct.

The Universe is like that. Most of it is a complete mess.

3 Responses to “Heart of Darkness”

  1. …and edge-effects in weak lensing maps are notoriously common.

    see a previous jee paper

    or my thesis…

  2. Bryn Jones Says:

    It’s a nice paper, full of careful work and really nice data, although it’s not entirely clear to me how well the distribution of the galaxies in the lensing clusters has been measured, or what the statistical uncertainties are. Certainly the structure of the clusters is complex. I wonder what additional clusters might be lurking in the background.

    We’ll have to see whether anybody else has an independent attempt at the problem in coming years and whether they reach similar conclusions.

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