19 September 2008

Milky Way ringed by ‘missing galaxies’

Cosmos Online
The Milky Way could be surrounded by up to 2,000 small galaxies, too faint to be seen with current technology, say astrophysicists.
Leo II dwarf spheroidal galaxy

Emerging picture: More than 10,000 times fainter than the Milky Way, the Leo II dwarf spheroidal galaxy is 500 light years across and is approximately 600,000 light years away from the Sun. Credit: Josh Simon using data made available by the Sloan Digital Sky Survey.

SYDNEY: The Milky Way could be surrounded by up to 2,000 small galaxies, too faint to be seen with current technology, say astrophysicists.

Their research has implications for understanding how galaxies form and the nature of dark matter – the invisible matter that pervades and surrounds all galaxies, including our own.

Ultra-faint dwarfs

Astronomers know of 24 satellite galaxies of the Milky Way, including the two bright Magellanic Clouds, which on dark nights are visible just to the side of the wide band of the Milky Way.

But the U.S. researchers, led by astrophysicist Erik Tollerud from the University of California, Irvine, say there are at least 400 and as many as 2,000 more satellite galaxies – we just haven’t spotted them yet because they are so faint.

“For every bright galaxy we can detect in the sky, there are likely hundreds of ultra-faint dwarf galaxies surrounding them that we are unable to detect with current technology,” University of California team member James Bullock told Cosmos Online. “It seems that these nearly-invisible galaxies are likely the most common galaxies in the universe.”

Dwarf galaxies are roughly spherical patches of stars that dwell in a halo of dark matter. Most are much smaller than the Magellanic Clouds, (which contain billions of stars) and contain from less than a thousand to hundreds of millions of stars.

Hidden from view

The galaxies are of interest because one of astronomy’s major theories, the Cold Dark Matter (CDM) theory, which very successfully explains how the universe formed at large scales, doesn’t fit for smaller galaxies.

The theory predicts that larger galaxies like our own should be ringed by hundreds of smaller galaxies. For ten years, astronomers have looked for evidence of these galaxies, or modified the theory to explain their absence.

“This has given rise to the idea that there are a bunch of satellite [galaxies] out there that are ‘missing’,” said Tollerud.

Recently, the largest ever survey of the sky, the Sloan Digital Sky Survey, has doubled the number of known dwarf galaxies.

The new study, published in the Astrophysical Journal, matched observations of nearby space made by the Sloan survey with predictions made by a supercomputer simulation of the distribution of dwarf galaxies in space.

Bullock likens it to counting cars on a road on a dark night. Only nearby car lights would be visible, but many more cars could be out there but too far away to be seen, especially if their lights were dim.

However, if you knew the approximate spacing between cars on the road, you could infer how many cars there were in total based on the few car lights that you could see, he said. “This is basically what we did — except that we were counting dwarf galaxies.”

Formation of galaxies

Tollerud said that the results have implications for our understanding of the formation of galaxies. The small size of these dwarf galaxies makes it easier to construct detailed scenarios about their formation in a way that’s impossible for big galaxies like the Milky Way, he said. “We can then generalise this to understand other types of galaxies.”

The experts also believe that the existence of 400 satellite galaxies may set a limit on how the fast the elusive particles that are dark matter can move, and hence how ‘warm’ dark matter can be.

“If [dark matter particles] are moving too quickly, they can’t clump up into subhalos [which contain dwarf galaxies]. So the existence of about 400 satellites means that the dark matter has to be moving slow enough to clump into at least 400 subhalos,” said Tollerud.

Nice and novel

Commenting on the research, astrophysicist Geraint Lewis, from the University of Sydney in Australia, said it was “novel” and a “nice prediction”. But he said the presence of these extra galaxies hinged on whether the CDM theory was correct and that he was only half convinced the missing galaxies would be found in future, deeper sky surveys.

“If we look up [to the sky] and see them that would be great and everyone would be happy. If we don’t see [the missing galaxies], people will still be running around trying to explain why they should still be there,” he said.

Larger sky surveys, such as those possible with the Australian National University’s SkyMapper telescope, which became operational in July this year, should reveal if the researchers are correct in their predications within five to ten years, added Lewis.


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