Saturday, April 21, 2012

No Dark Matter In Our Neighborhood?

{Don't miss our nomination period to nominate your most attractive physicists}

The most provocative report from this past week is the recent publication of a new result that tries to find any sign of dark matter in our part of the galaxy. Strangely enough, the researchers found none, or at least, there wasn't any need for the presence of dark matter to explain the observed celestial dynamics.

But that's not what Christian Moni Bidin, an astronomer at the University of ConcepciĆ³n in Chile, and colleagues find. Using data gathered with several telescopes, they studied old stars called red giants in a cylindrical region a couple of light-years wide and extending 13,000 light-years above the plane of the galaxy. Treating the stars a bit like atoms in a gas, researchers assumed that they were trapped in the gravitational "well" of the galaxy. So by studying distributions of the stars' speeds in three dimensions, they could deduce the well's shape and hence the total distribution of mass from both dark and ordinary matter along the cylinder. Subtracting the distribution of ordinary matter as determined from star counts would then reveal the distribution of dark matter.

When Moni Bidin and colleagues did the analysis, however, they found that no dark matter was needed to explain the stars' speeds. The researchers had expected to detect a complicated mass distribution with a contribution from the galaxy's disk of stars and gas and the presumably spherical "halo" of dark matter surrounding the disk. Instead, they found that the disk alone neatly explained their data, as they report in a paper in press at The Astrophysical Journal.
Of course, there are skepticism with this report, especially with regards to the type of analysis being done.

Or not. The new result may say more about the method than the distribution of dark matter, Navarro says. To get that distribution, at each position in space Moni Bidin and colleagues must subtract one large quantity (the amount of ordinary matter) from another large quantity (the amount of total mass) to get a small quantity. That process is likely to suffer from large uncertainties, Navarro says. "I applaud them for trying," he says. "I just don't think this method will ever give a conclusive answer." Moni Bidin says the method is robust and that larger surveys to come will pin down the dark matter distribution more precisely.
As with any report that are this controversial (re: superluminal neutrinos), we need to let the system works out on its own. A lot more studies need to be done with more analysis and observations. Things are seldom confirmed with just one observation using one technique, especially in a situation such as this where the methodology of analysis influence the result. So I certainly would not want to draw any kind of conclusion this soon.


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