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

Half of all exoplanet host stars are binaries

The Kepler field of view, located between two bright stars in the summer triangle, rising over the WIYN 3.5m telescope in southern Arizona.

Imagine living on an exoplanet with two suns. One, you orbit and the other is a very bright, nearby neighbour looming large in your sky.

With this "second sun" in the sky, nightfall might be a rare event, perhaps only coming seasonally to your planet. A new study suggests that this could be far more common than we realized.

The NASA Kepler Space Telescope has confirmed about 1000 exoplanets, as well as thousands more stars considered "Kepler objects of interest", dubbed KOIs, stars that could possibly host planets.

Until now, there has been an unanswered question about exoplanet host stars; how many host stars are binaries?

Binary stars have long been known to be commonplace, about half the stars in the sky are believed to consist of two stars orbiting each other.

So, are stars with planets equally likely to have a companion star, or do companion stars affect the formation of planets?

A team of astronomers, led by Dr. Elliott Horch, Southern Connecticut State University, have shown that stars with exoplanets are just as likely to have a binary companion: that is, 40% to 50% of the host stars are actually binary stars.

As Dr. Horch said, "It's interesting and exciting that exoplanet systems with stellar companions turn out to be much more common than was believed even just a few years ago."

Their study makes use of very high spatial resolution observations that were carried out on the WIYN 3.5m telescope located on Kitt Peak in southern Arizona and the Gemini North telescope located on Mauna Kea in Hawaii.

The technique used by the team is called speckle imaging and consists of obtaining digital images of a small portion of the sky surrounding a star of interest, 15 to 25 times a second.

The images are then combined in software using a complex set of algorithms, yielding a final picture of the star with a resolution better that that of the Hubble space telescope.

By using this technique, the team can detect companion stars that are up to 125 times fainter than the target, but only 0.05 arcseconds away.

For the majority of the Kepler stars, this means companion stars with a true separation of a few to about 100 times the Sun-Earth distance (termed the astronomical distance, or AU).

By noting the occurrence rate of these true binary companion stars, the discoveries can be extended to show that half of the stars that host exoplanets are probably binaries.

Artist’s concept of exoplanets in a two-stars system. Credit: NASA/JPL-Caltech/T. Pyle

Co-author of the study, Dr. Steve B. Howell (NASA Ames Research Center), commented, "An interesting consequence of this finding is that in the half of the exoplanet host stars that are binary we can not, in general, say which star in the system the planet actually orbits."

Steve B. Howell

Kepler has discovered a number of circumbinary planets, that is, a planet that orbits both stars in very close binary systems.

There also exist exoplanets that are known to orbit one of the stars in very wide binary systems.

If the two stars are very close to each other and the planet far away, a circumbinary planet will be reminiscent of Tatooine in Star Wars.

If instead the exoplanet orbits one of the stars in a very wide pair, the companion star might appear simply as a bright star among others in the night sky.

"Somewhere there will be a transition between these two scenarios," Howell said," but we are far from knowing where."

In a study like this, it is critical to rule out faint companions that are only in the line of sight with the KOI star.

To allow for these possibilities, the team performed a model simulation that relied on known statistical properties of binary star systems and line of sight companions.

The results suggest that the large majority of the stellar companions to KOIs are true bound companions, not line of sight stars unconnected with the system.

More Information: "LIMITS ON STELLAR COMPANIONS TO EXOPLANET HOST STARS WITH ECCENTRIC PLANETS" has been accepted for publication in the Astrophysical Journal: Authors - Stephen R. Kane et al. 2014 ApJ 785 93 doi:10.1088/0004-637X/785/2/93.

Flooding: Half of Venice underwater

The city has for years been wrestling with the problems posed by the threat of rising sea levels. Last year local authorities confirmed they were looking at a scheme to raise the city's buildings to meet the problem.

Much of the historic Italian city of Venice, including St. Mark's Square, was underwater Monday following a meteorological depression combined with natural tide waters, officials said.

The tide monitoring centre said 45 percent of the Renaissance city was swamped when the lagoon rose 131 centimetres (more than four feet).

Venice was flooded 50 times between 1993 and 2002, with the worst 'acqua alta' on November 4, 1966, when the city was submerged by 1.94 metres of water amid catastrophic flooding throughout the country.

In February 1986, levels reached 1.58 metres above normal, and in December 2008 waters surged 1.56 metres.

The city has for years been wrestling with the problems posed by the threat of rising sea levels. Last year local authorities confirmed they were looking at a scheme to raise the city's buildings to meet the problem.

Under Operation "Rialto", local officials and engineers were looking at using piston-supported-poles placed at the bottom of each structure to lift buildings by up to a metre.

In April 2007, the United Nations cultural organisation UNESCO warned that Venice was one of its designated World Heritage sites that was threatened by climate change.