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13th October 2014

Half as much dark matter in Milky Way galaxy as previously thought

New measurements reveal there is half as much dark matter in our galaxy as previously thought, solving the 15-year-old "missing satellite galaxy" problem.

 

milky way galaxy
Credit: ESO/L. Calçada

 

New measurements of dark matter in our own Milky Way galaxy reveal there is half as much of the mysterious substance as previously thought. Astronomers from the International Centre for Radio Astronomy Research (ICRAR) used a method developed almost 100 years ago to discover that the weight of dark matter in our galaxy is 800 billion (or 8 x 1011) times the mass of the Sun. They probed the edge of the Milky Way, looking closely, for the first time, at the fringes about 5 million trillion kilometres from Earth.

Astrophysicist Dr Prajwal Kafle said we have known for a while that most of the Universe is hidden:“Stars, dust, you and me, all the things that we see, only make up about 4 per cent of the entire Universe. About 25 per cent is dark matter and the rest is dark energy.”

Dr Kafle was able to measure the mass of the dark matter in the Milky Way by studying the speed of stars throughout the galaxy, including the edges, which had never been studied in this detail before. He used a robust technique developed by British astronomer James Jeans in 1915 – decades before the discovery of dark matter. This new calculation helps to solve a mystery that has been haunting theorists for almost two decades.

“The current idea of galaxy formation and evolution – called the Lambda Cold Dark Matter theory – predicts that there should be a handful of big satellite galaxies around the Milky Way that are visible with the naked eye, but we don’t see that,” Dr Kafle said. “When you use our measurement of the mass of dark matter, the theory predicts that there should only be three satellite galaxies out there, which is exactly what we see; the Large Magellanic Cloud, the Small Magellanic Cloud and the Sagittarius Dwarf Galaxy.”

 

 

 

University of Sydney astrophysicist Prof. Geraint Lewis, who was also involved in the research, said the missing satellite problem had been “a thorn in the cosmological side for almost 15 years.”

“Dr Kafle’s work has shown that it might not be as bad as everyone thought, although there are still problems to overcome," he said.

The study also presented a holistic model of the Milky Way, which allowed the scientists to calculate several interesting factors, such as the speed required to leave the galaxy.

“Be prepared to hit 550 kilometres per second if you want to escape the gravitational clutches of our galaxy,” Dr Kafle said. “A rocket launched from Earth needs just 11 kilometres per second to leave its surface.”

 

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