Half as much dark matter as astronomers thought

The Milky Way contains half as much dark matter than previously thought according to new research which has weighed the mysterious substance. Providing the precise mass of dark matter in the Milky Way has proved problematic, but based on expectations from computer models, scientists had settled on a value of between two and four thousand billion times the mass of the sun. Using a method developed almost a century ago, astronomers from the International Centre for Radio Astronomy Research (ICRAR) discovered the weight of dark matter in the galaxy is 8 x 10¹¹ times the mass of the Sun – more than a factor of two below the previously expected value. “Stars, dust, you and me, all the things we see, only make up about 4% of the entire Universe,” said Dr Prajwal Kafle, a research associate at ICRAR. “About 25% is dark matter and rest is dark energy.” Kafle used a technique developed by British physicist Sir James Jeans in 1915 to measure the mass of dark matter right up to the edges of the Milky Way – about 5 million billion kilometres from Earth. “The idea is fairly simple: you observe stars with a telescope and collect vital information such as their motion around the centre of the Galaxy,” Kafle told Laboratory News. “Those stars are in a periodic orbit within the Galaxy. It means something is holding them in an orbit; otherwise they will just fly apart. What could it be? Answer is obviously gravity. Then we ask what source of gravity is holding these stars.  We count the stars, multiply them with average weight of a star and come up with total force which is never quite enough, actually way less than the amount required to hold such distant stars in a continuous motion. It means there must be something invisible (strictly speaking, non-interacting except gravitationally), that is what we call the dark matter. In brief, the speed of the distant stars tells us how much matter there must be within the Galaxy.” http://youtu.be/kXstoiyip9Y

This animation shows a supercomputer simulation of a galaxy like the Milky Way and its invisible dark matter halo. Credit: Prof Chris Power and Dr Rick Newton, ICRAR. Music by Reuben Christman (www.reubenchristman.com).

Kafle’s measurements help solve a mystery which has irritated theorists for the last 20 years - the number of dwarf galaxies in the Milky Way. “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,” he said, “but we don’t see that.” “When you use our measurement of the mass of the 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.” The study, published in Astrophysics Journal, has presented a holistic model of our galaxy which allows scientists to measure several things, including the speed needed to leave the galaxy. “Be prepared to hit 550km per second if you want to escape the gravitational clutches of our galaxy,” Kafle said. “A rocket launched from Earth needs just 11km per second to leave its surface.” On the Shoulders of Giants: Properties of the Stellar Halo and the Milky Way Mass Distribution