Wednesday, March 6, 2013

Dark Matter in 3D

Abell 383 is a galaxy cluster located approximately 2.3 billion light-years away. Galaxy clusters are the largest celestial objects gravitationally bound; they play a key role in understanding dark matter. Astronomers recently used data from Chandra X-ray Observatory to map the cluster’s dark matter. After careful data analysis they were able to determine where the dark matter was located and how it is distributed.

The dark matter was observed through gravitational effects and the data shows there is six times as much dark matter as normal matter. Astronomers gathered the data and were able to generate 3-D pictures of the elusive dark matter. They observed the dark matter was stretched out to form a gigantic American “football” and that the football’s point is lines up close to the line of sight.

Two teams of scientists at two different universities studied Abell 383 and came up with similar as well as very different observations. After combining x-ray observations of “normal matter” with optical data derived from gravitational lensing, they observed arc-like appearance for some of the galaxies. You can see the “arc” in the image. The vast purple area is x-ray data from Chandra highlighting hot gas – the dominate type of matter in the cluster.

Team Newman (and yes I said that in Jerry Seinfeld’s voice), headed up by Andrew Newman (CalTech) and Tommaso Treu (UCSB) combined Hubble lensing data with Japanese telescope Subaru and adding Keck observations to measure star velocities in cluster center. This allowed for a direct estimate of dark matter. The data showed the dark matter was not as concentrated in the cluster’s center as predicted by the standard cold dark matter model.

Team Morandi lead by Andrea Morandi (Tel Aviv University) and Marceau Limosin (Université de Provence/University of Copenhagen) determined the dark matter concentration to be higher towards the cluster center and the observation are in line with most theoretical models. Data from the Hubble space telescope was also analyzed by this team.

So we have two teams and two different results – how does this happen? One key reason is Team Newman used velocity data in the central galaxy and was able to estimate the dark matter density close to the cluster’s center – as close as 6,500 light-years away from center. Without the velocity data, Team Morandi was only able to estimate density as close as 80,000 light-years. Team Morandi were able to predict the dark matter “football” orientation. Further studies are needed to clear up the discrepancies between these two teams. It may be a while before we find out which is correct – Team Newman or Team Morandi.

If Team Newman’s results are validated and there is a lack of dark matter in Abell 383’s center, then we need a better understanding of how normal matter conducts itself in galactic centers and may demonstrate how dark matter particles interact.


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