Plenty of Dark Matter near Solar System

A study led by the University of Zürich and the ETH Zürich has revealed the presence of large amounts of invisible ‘dark matter’ near the Solar System.

The astronomers have used the high resolution simulation of the Milky Way Galaxy to test their mass-measuring technique (A. Hobbs / The University of Zürich)

The results are consistent with the theory that the Milky Way Galaxy is surrounded by a massive ‘halo’ of dark matter, but this is the first study of its kind to use a method rigorously tested against mock data from high quality simulations.

Dark matter was first proposed by the Swiss astronomer Fritz Zwicky in the 1930s. He found that clusters of galaxies were filled with a mysterious dark matter that kept them from flying apart. At nearly the same time, Jan Oort in the Netherlands discovered that the density of matter near the Sun was nearly twice what could be explained by the presence of stars and gas alone. In the intervening decades, astronomers developed a theory of dark matter and structure formation that explains the properties of clusters and galaxies in the Universe, but the amount of dark matter in the solar neighborhood has remained more mysterious.

For decades after Oort’s measurement, studies found 3-6 times more dark matter than expected. Then last year new data and a new method claimed far less than expected. The community was left puzzled, generally believing that the observations and analyses simply weren’t sensitive enough to perform a reliable measurement.

Now the researchers have developed a new technique to determine the local matter and dark matter densities from the kinematics of K dwarf stars near the Sun.

They used a state-of-the-art simulation of the Milky Way to test their mass-measuring method before applying it to real data. This threw up a number of surprises. They found that standard techniques used over the past twenty years were biased, always tending to underestimate the amount of dark matter. The researchers then developed a new unbiased technique that recovered the correct answer from the simulated data. By applying their technique to the positions and velocities of thousands of orange K dwarf stars in the solar neighborhood, they obtained a new measure of the local dark matter density.

“We are 99% confident that there is dark matter near the Sun,” said Dr Silvia Garbari, a lead author of the study accepted for publication in the Monthly Notices of the Royal Astronomical Society. In fact, if anything, the authors’ favored dark matter density is a little high.  With 90% confidence, they find more dark matter than expected.

“It could be the first evidence for a ‘disc’ of dark matter in our Galaxy, as recently predicted by theory and numerical simulations of galaxy formation. Or it could be that the dark matter halo of our Galaxy is squashed, boosting the local dark matter density,” she said.

Many researchers are placing their bets on dark matter being a new fundamental particle that interacts only very weakly with normal matter, but strongly enough to be detected in experiments deep underground.

“An accurate measure of the local dark matter density is vital for such experiments,” said Prof George Lake. “If dark matter is a fundamental particle, billions of these particles will have passed through your body by the time you finish reading this article. Experimental physicists hope to capture just a few of these particles each year in experiments like XENON and CDMS currently in operation. Knowing the local properties of dark matter is the key to revealing just what kind of particle it consists of.”

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Bibliographic information: Garbari S et al. 2012. A new determination of the local dark matter density from the kinematics of K dwarfs. Accepted for publication in Monthly Notices of the Royal Astronomical Society; arXiv: 1206.0015G