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Getting to know the galactic neighbors

January 10, 2007 By Alexandra Witze This article courtesy of Nature News.

Astronomers make startling discoveries in our own back yard.

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Astronomers are beginning to realize that we don't know our cosmic neighbourhood very well after all. Some of the galaxies next door to our very own Milky Way are speeding past us so fast that they threaten to rewrite the textbooks, whereas others are so teeny that they may deserve the entirely new name of 'hobbit' galaxies.

One might think that with all our knowledge of the far fringes of the Universe, that our own neighbourhood had been pretty well mapped. But presentations at the American Astronomical Society meeting in Seattle this week have shown that there is still plenty of room for surprises.

The Milky Way's largest and most famous immediate neighbours the satellite galaxies known as the Large and Small Magellanic Clouds sit some 160,000 light-years (1.5 billion billion kilometres) away from us. They have long been thought to be gravitationally bound both to each other and to the Milky Way. But Nitya Kallivayalil, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, told the meeting: "A lot of the fundamental things we thought we knew about them have to be reassessed."

Her team used the Hubble Space Telescope to measure, more carefully than ever before, how the Magellanic Clouds move through space. The data suggest that they are moving nearly twice as fast as astronomers previously thought.

That could indicate that they aren't bound to the Milky Way at all, and may one day zoom off to another corner of the Universe. If they are bound, Kallivayalil says, the Milky Way must be much more massive than astronomers currently think it is. Either explanation would be cause for rewriting textbooks, she says.

Hobbits

A second surprising find was presented by Daniel Zucker, an astronomer at Cambridge University, UK.

Zucker and his team have spotted at least seven new, tiny galaxies in our neighbourhood, using the New-Mexico-based telescope of the Sloan Digital Sky Survey. These add to roughly a dozen other 'dwarf' galaxies that had been previously seen orbiting the Milky Way.

Leading models had predicted that on the order of 100 dwarf galaxies should be orbiting the Milky Way, so the new finds help to make up some of the shortfall, says Zucker. But the odd thing is how small these galaxies are. "They seem to be much fainter than anyone had suspected galaxies could be," he says. They are so small that he argues for the new if unofficial designation of 'hobbits'.

The most peculiar of these, spotted just last month, is called Leo T. Despite being an entire galaxy, it is only about as bright as 50,000 Suns (other dwarf galaxies are on the order of hundreds of thousands of Suns, compared with 1 trillion Suns for the Milky Way).

"We find this very exciting," says Zucker. Could the cosmic neighbourhood, he asks, "be filled with objects like this?"

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