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

First Thorne-Żytkow object discovery, a bizarre type of hybrid star

Credit: A. Fujii

In a discovery decades in the making, scientists have detected the first of a "theoretical" class of stars first proposed in 1975 by physicist Kip Thorne and astronomer Anna Żytkow.

Thorne-Żytkow objects (TŻOs) are hybrids of red supergiant and neutron stars that superficially resemble normal red supergiants, such as Betelguese in the constellation Orion.

They differ, however, in their distinct chemical signatures that result from unique activity in their stellar interiors.

TŻOs are thought to be formed by the interaction of two massive stars―a red supergiant and a neutron star formed during a supernova explosion, in a close binary system.

While the exact mechanism is uncertain, the most commonly held theory suggests that, during the evolutionary interaction of the two stars, the much more massive red supergiant essentially swallows the neutron star, which spirals into the core of the red supergiant.

While normal red supergiants derive their energy from nuclear fusion in their cores, TŻOs are powered by the unusual activity of the absorbed neutron stars in their cores.

The discovery of this TŻO thus provides evidence of a model of stellar interiors previously undetected by astronomers.

Project leader Emily Levesque of the University of Colorado Boulder, who earlier this year was awarded the American Astronomical Society's Annie Jump Cannon Award, said, "Studying these objects is exciting because it represents a completely new model of how stellar interiors can work."

"In these interiors we also have a new way of producing heavy elements in our universe. You've heard that everything is made of 'star stuff', inside these stars we might now have a new way to make some of it."

The study, accepted for publication in the Monthly Notices of the Royal Astronomical Society Letters, is co-authored by Philip Massey, of Lowell Observatory in Flagstaff, Arizona; Anna Żytkow of the University of Cambridge in the U.K.; and Nidia Morrell of the Carnegie Observatories in La Serena, Chile.

The astronomers made their discovery with the 6.5-meter Magellan Clay telescope on Las Campanas, in Chile.

They examined the spectrum of light emitted from apparent red supergiants, which tells them what elements are present.

When the spectrum of one particular star, HV 2112 in the Small Magellanic Cloud, was first displayed, the observers were quite surprised by some of the unusual features.

Morrell explained, "I don't know what this is, but I know that I like it!"

When Levesque and her colleagues took a close look at the subtle lines in the spectrum they found that it contained excess rubidium, lithium and molybdenum.

Past research has shown that normal stellar processes can create each of these elements. But high abundances of all three of these at the temperatures typical of red supergiants is a unique signature of TŻOs.

"I am extremely happy that observational confirmation of our theoretical prediction has started to emerge," Żytkow said. "Since Kip Thorne and I proposed our models of stars with neutron cores, people were not able to disprove our work."

"If theory is sound, experimental confirmation shows up sooner or later. So it was a matter of identification of a promising group of stars, getting telescope time and proceeding with the project."

The team is careful to point out that HV 2112 displays some chemical characteristics that don't quite match theoretical models. Massey points out, "We could, of course, be wrong.

There are some minor inconsistencies between some of the details of what we found and what theory predicts but the theoretical predictions are quite old, and there have been a lot of improvements in the theory since then. Hopefully our discovery will spur additional work on the theoretical side now."

More information: Read the full study at arxiv.org/abs/1406.0001

ESA's XMM-Newton: Pulsar encased in a supernova bubble

Credit: ESA /XMM-Newton /L. Oskinova /M. Guerrero; CTIO /R. Gruendl / Y.H. Chu

Massive stars end their lives with a bang: exploding as spectacular supernovas, they release huge amounts of mass and energy into space.

These explosions sweep up any surrounding material, creating bubble remnants that expand into interstellar space.

At the heart of bubbles like these are small, dense neutron stars or black holes, the remains of what once shone brightly as a star.

Since supernova-carved bubbles shine for only a few tens of thousands of years before dissolving, it is rare to come across neutron stars or black holes that are still enclosed within their expanding shell.

Pulsar SXP 1062

This image captures such an unusual scene, featuring both a strongly magnetised, rotating neutron star – known as a pulsar – and its cosmic cloak, the remains of the explosion that generated it.

Small Magellanic Cloud

This pulsar, named SXP 1062, lies in the outskirts of the Small Magellanic Cloud, one of the satellite galaxies of our Milky Way galaxy.

It is an object known as an X-ray pulsar: it hungrily gobbles up material from a nearby companion star and burps off X-rays as it does so.

In the future, this scene may become even more dramatic, as SXP 1062 has a massive companion star that has not yet exploded as a supernova.

Most pulsars whirl around incredibly quickly, spinning many times per second.

However, by exploring the expanding bubble around this pulsar and estimating its age, astronomers have noticed something intriguing: SXP 1062 seems to be rotating far too slowly for its age. It is actually one of the slowest pulsars known.

While the cause of this weird sluggishness is still a mystery, one explanation may be that the pulsar has an unusually strong magnetic field, which would slow the rotation.

The diffuse blue glow at the centre of the bubble in this image represents X-ray emission from both the pulsar and the hot gas that fills the expanding bubble.

The other fuzzy blue objects visible in the background are extragalactic X-ray sources.

This image combines X-ray data from ESA's XMM-Newton (shown in blue) with optical observations from the Cerro Tololo Inter-American Observatory in Chile.

The optical data were obtained using two special filters that reveal the glow of oxygen (shown in green) and hydrogen (shown in red).

The size of the image is equivalent to a distance of 457 light-years on a side.

This image was first published in 2011.

More information: "Discovery of a Be/X-ray pulsar binary and associated supernova remnant in the Wing of the Small Magellanic Cloud." V. Hénault-Brunet, L. M. Oskinova, M. A. Guerrero, et al. Monthly Notices of the Royal Astronomical Society: Letters 420 (1) L13 (2012) DOI: 10.1111/j.1745-3933.2011.01183.x

NGC 602: Under the 'wing' of the Small Magellanic Cloud (SMC)

New Chandra observations have been used to make the first detection of X-ray emission from young stars with masses similar to our Sun outside our Milky Way galaxy. 

The Chandra observations of these low-mass stars were made of the region known as the "Wing" of the Small Magellanic Cloud (SMC), one of the Milky Way's closest galactic neighbors. 

In this composite image of the Wing the Chandra data is shown in purple, optical data from the Hubble Space Telescope is shown in red, green and blue and infrared data from the Spitzer Space Telescope is shown in red. 

Astronomers call all elements heavier than hydrogen and helium -- that is, with more than two protons in the atom's nucleus -- "metals." 

The Wing is a region known to have fewer metals compared to most areas within the Milky Way. 

The Chandra results imply that the young, metal-poor stars in NGC 602a produce X-rays in a manner similar to stars with much higher metal content found in the Orion cluster in our Galaxy. 

Credit: X-ray: NASA /CXC /Univ.Potsdam /L.Oskinova et al; Optical: NASA/STScI; Infrared: NASA/JPL-Caltech

The Small Magellanic Cloud (SMC) is one of the Milky Way's closest galactic neighbors. Even though it is a small, or so-called dwarf galaxy, the SMC is so bright that it is visible to the unaided eye from the Southern Hemisphere and near the equator.

Many navigators, including Ferdinand Magellan who lends his name to the SMC, used it to help find their way across the oceans.

Modern astronomers are also interested in studying the SMC (and its cousin, the Large Magellanic Cloud), but for very different reasons.

Because the SMC is so close and bright, it offers an opportunity to study phenomena that are difficult to examine in more distant galaxies.