Scientists New Estimate of Dark Matter Half previous estimate

Artist’s impression of the Milky Way and its dark matter halo (shown in blue, but in reality invisible). 

Credit: ESO/L. Calçada

A new measurement of dark matter in the Milky Way has revealed there is half as much of the mysterious substance as previously thought.

Australian astronomers used a method developed almost 100 years ago to discover that the weight of dark matter in our own galaxy is 800 000 000 000 (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 of the galaxy about 5 million billion kilometres from Earth.

Astrophysicist Dr Prajwal Kafle, from The University of Western Australia node of the International Centre for Radio Astronomy Research (ICRAR), 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," he said.

"About 25 per cent is dark matter and the rest is dark energy."

Dr Kafle, who is originally from Nepal, 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 to this detail before.

He used a robust technique developed by British astronomer James Jeans in 1915, decades before the discovery of dark matter.

Dr Kafle's measurement 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 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."

University of Sydney astrophysicist Professor 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 measure several interesting things 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, which is already about 300 times faster than the maximum Australian speed limit in a car!"

More information: 'On the Shoulders of Giants: Properties of the Stellar Halo and the Milky Way Mass Distribution' P. R. Kafle, S. Sharma, G. F. Lewis, and J. Bland-Hawthorn. Published in the Astrophysical Journal October 10th, 2014. Available at: iopscience.iop.org/0004-637X/794/1/59/. arxiv.org/abs/1408.1787

Outside the Milky Way: Globular cluster Messier 54

This new image from the ESO VLT Survey Telescope in northern Chile shows a vast collection of stars, the globular cluster Messier 54. 

This cluster looks similar to many others but it has a secret.

Messier 54 doesn't belong to the Milky Way, but is part of a satellite galaxy, the Sagittarius Dwarf Galaxy.

This parentage allowed astronomers to test whether there are also low levels of the element lithium in stars outside the Milky Way.

The Milky Way galaxy is orbited by more than 150 globular star clusters, which are balls of hundreds of thousands of old stars dating back to the formation of the galaxy.

One of these, along with several others in the constellation of Sagittarius (The Archer), was found in the late eighteenth century by the French comet hunter Charles Messier and given the designation Messier 54.

For more than two hundred years after its discovery Messier 54 was thought to be similar to the other Milky Way globulars, but in 1994 it was discovered that it was actually associated with a separate galaxy, the Sagittarius Dwarf Galaxy.

It was found to be at a distance of around 90 000 light-years, more than three times as far from Earth as the galactic centre.

Astronomers have now observed Messier 54 using the VLT as a test case to try to solve one of the mysteries of modern astronomy, the lithium problem.

Most of the light chemical element lithium now present in the Universe was produced during the Big Bang, along with hydrogen and helium, but in much smaller quantities.

Astronomers can calculate quite accurately how much lithium they expect to find in the early Universe, and from this work out how much they should see in old stars, but the numbers don't match, there is about three times less lithium in stars than expected.

This mystery remains, despite several decades of work.

Up to now it has only been possible to measure lithium in stars in the Milky Way, but now a team of astronomers led by Alessio Mucciarelli (University of Bologna, Italy) has used the VLT to measure how much lithium there is in a selection of stars in Messier 54.

They find that the levels are close to those in the Milky Way. So, whatever it is that got rid of the lithium seems not to be specific to the Milky Way.

This new image of the cluster was created from data taken with the ESO VLT Survey Telescope (VST) at the Paranal Observatory.

As well as showing the cluster itself it reveals the extraordinarily dense forest of much closer Milky Way stars that lie in the foreground.

More information: This research was presented in a paper, "The cosmological Lithium problem outside the Galaxy: the Sagittarius globular cluster M54", by A. Mucciarelli et al., to appear in Monthly Notices of the Royal Astronomical Society (Oxford University Press). (PDF)