The Milky Way’s Halo -A Relic of Smaller Ancient Galaxies?

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"We still have a lot to understand."

Masashi Chiba of Japan's Tohoku University.

If gazing up at the Milky Way can make you feel dizzy, it could be because the outer galaxy has been discovered to be a mix of two distinct components rotating in opposite directions. The Milky Way's main disk, home to our sun, rotates at an average speed of 500,000 mph. Surrounding the disk is what's now called the inner halo, which orbits in the same direction at about 50,000 mph. The thinly populated region, outer halo, spins in the opposite direction at roughly 100,000 mph.

The discovery was made by a team of international astronomers including Daniela Carollo, a researcher at Italy's Torino Observatory now undertaking her PhD at Australia's Mount Stromlo Observatory and Australia National University Astronomer Professor, John Norris. They made the discovery after studying 20,000 stars observed by the Sloan Digital Sky Survey (SDSS-II).

The finding proves what astronomers have suspected for around 30 years — that the Milky Way halo was assembled over time swallowing up smaller galaxies.

"By examining the motions and chemical makeup and the stars, we can see that the inner and outer halos are quite different beasts and they probably formed in different ways at different times," said Ms Carollo, lead author of the report.

"Although it was once considered a single structure, an analysis of the stars from SDSS-II shows that the halo is clearly divisible into two, broadly overlapping components. The discovery gives us a much clearer picture of the formation of the first objects in our Galaxy and in the entire Universe," she added.

"This shows conclusively that there are two major parts of the halo structure — inner and outer," Professor Norris said. "The inner halo is a part of the galaxy that is rotating slowly. The part where we live — the Galactic disc — gallops around at quite a rate, while the inner component is going in the same direction, but much more slowly. The outer halo rotates, albeit slowly, in the opposite direction."

The discovery also reveals the two halo components have different chemical compositions. Indeed, according to Carollo, inner-halo stars contain three times more heavy elements than outer-halo stars.
Professor Norris has spent more than two decades searching for the most chemically primitive stars in the galaxy. These fossils of the early universe are extremely rare, Norris explained, so finding them remains a classic "needle in a haystack problem". But he added that the discovery of a chemically distinct outer halo, "gives us a much better way to search the haystack."

Astronomers are now finding faint, dwarf galaxies in the halo of the Milky Way using advanced techniques and instruments at The University of Arizona/Smithsonian 6.5-meter MMT Observatory at Mount Hopkins, Ariz.

"These are galaxies that might contain as few as a thousand stars, and those stars are being pulled out into the halo of our Milky Way," said UA astronomer Ed Olszewski."We're trying to understand whether these unbelievably faint objects are intact or have been mostly pulled apart by the Milky Way. We're trying to understand what the halo of the Milky Way really looks like, how many of these objects are in the halo, and whether our census of the population in the halo agrees or conflicts with the cosmological models.

"Knowing how many of these incredibly puny satellite galaxies populate our galactic neighborhood is important if we are to know whether cosmological models used to describe the evolution of the structure of galaxies are correct or way off base," he added.

"The sorts of objects we're finding have so few stars that one might think they're not galaxies at all, except that their internal motions imply that, unlike star clusters, they contain dark matter just like big galaxies do," Olszewski said.

A more accurate census of very faint, local dwarf galaxies is important because it will help scientists determine how much dark matter they might contain, he said. Scientists believe that "dark matter," or matter that is observed only by the effects of gravity but cannot be seen otherwise because it emits no radiation, makes up about 25 percent of the universe. "Normal" matter is thought to make up between 2 percent and 4 percent of the universe, with the remaining bulk being dark energy. 

Posted by Casey Kazan.

Sources:
http://groups.google.com/group/sci.astro/browse_thread/thread/2a12300764684a5dhttp://www.space.com/scienceastronomy/071212-milky-way-halo.html
http://uanews.org/node/25687

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