Friday, January 28, 2011

Dark Matter Reconstruction From Radio Experiments.

As photons move through the universe they get gravitationally lensed as the pass by large clumps of matter. (As shown in the image above.) Dark matter, being the dominant form of matter, lenses these photons more than anything.  Therefore, by studying the lensing properties of incoming photons, in principle we can reconstruct what the profiles of the dark matter doing that lensing.

Now, put (hopefully) more simply: we can't directly see dark matter.  But taking the statistical lensing properties of the incoming photons we can hopefully reconstruct what the dark matter looks like.  In this way we can "see" the clumps of dark matter in the universe directly.

Recently, Brown and Battye have proposed a new method for reconstructing the projected dark matter distribution using radio surveys like SKA and e-MERLIN. They then test their method on simulated data as discussed below.

The plot above shows a mock initial dark matter distribution from which they make simulations of the kind of data e-MERLIN would see coming from such a distribution of dark matter.  If their method works, this is the distribution they will reconstruct.
This next plot above shows how well they are able to reconstruct the input dark matter distribution.  As can be seen, the main features, especially the two dominant clusters, can be resolved fairly well.  (Especially given the input data is smoothed by the beam of the instrument and so this at some level is approaching as good as things get for a single experiment.)

Punchline: So, using this lensing reconstruction technique for be can begin to "see" what the underlying dark matter clumps look like.

I for one am excited about where lensing is headed.  Utilizing a variety of lensing techniques, including the method for radio sources done here, we may one day reconstruct the dark matter throughout the universe with great precision, especially when we combine the data from many experiments at many wavelengths.  Again, in this way we can visually "see" the dark matter that comprises our universe.
Michael L. Brown, & Richard A. Battye (2011). Mapping the dark matter with polarized radio surveys E-Print arXiv: 1101.5157v1

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