Sunday, August 19, 2007

Dark matter 3

This pre-print tells the story of Abell 520, a galaxy cluster formed by the collision of several smaller clusters.

In many respects it is an ordinary cosmic train wreck.  The original clusters are still grouped together.  The big, fluffy gas clouds collided and ground to a halt, being left behind by the stars.  The gas was heated in the process and can now be seen glowing x-ray hot in the gulfs between the clusters.  Gravitational lensing of distant background galaxies shows that Abell 520 has the usual ratio of dark mass to stars, about 4:1.

But the dark matter has a surprising location:  like the gas, it too is separated from the galaxies in the vast gulfs between them.  Dark matter had been thought to not interact with itself much, as shown by the Bullet cluster of galaxies, and also thought to start out as a spherical halo around ordinary galaxies.  It is mysterious and surprising for the dark matter to be so thoroughly separated from the stars.

I see two possibilities.  One is that dark matter can interact with itself, and Abell 520's dark matter has an unusual density or composition that facilitates this.  The other is that the Big Bang managed to synthesize the dark matter and ordinary matter at different locations.  For whatever reasons, they are usually created together, but in, say, an unusual primordial swirl, they need not be.

I lean towards the second explanation.  We have already discovered a galaxy, VIRGOHI21, that is 99.9% dark matter.  It is only a leap of scale to imagine that happening to a whole cluster.

I am very curious to see rotation curves for some of the galaxies in question, to verify that they are dark matter depleted.

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