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. Astronomers Find Two New Milky Way Companions

Images of the new dwarfs discovered in the constellations of Canes Venatici (left, see paper by Zucker et al.) and Bootes (right, see paper by Belokurov et al.) Image credit: SDSS/Cambridge University
by Staff Writers
Cambridge, United Kingdom (SPX) May 10, 2006
British astronomers said Monday they have discovered two new, very faint companion galaxies to the Milky Way. The first was found in the direction of the constellation Canes Venatici, or the Hunting Dogs, by Daniel Zucker of Cambridge University.

His colleague Vasily Belokurov discovered the second in the constellation Bootes, or the Herdsman. Both astronomers used the second Sloan Digital Sky Survey, which covers more than one-quarter of the sky.

"I was poring over the survey's map of distant stars in the Northern Galactic sky - what we call a Field of Streams -- and noticed an overdensity in Canes Venatici," Zucker said. "Looking further, it proved to be a previously unknown dwarf galaxy."

He said the dwarf is located about 640,000 light-years away, making it one of the most remote of the Milky Way's companions.

Zucker said he e-mailed Belokurov with the news, and Belokurov responded a few hours later with the discovery of an even fainter dwarf galaxy.

The new galaxy in Bootes, which Belokurov named Boo, shows a distorted structure that suggests it is being disrupted by the Milky Way's gravitational tides. "Something really bashed Boo about," he said.

Although the dwarf galaxies are relatively nearby, they nevertheless have been difficult to detect because they are so dim. The new galaxy in Bootes is the faintest galaxy discovered so far, with a total luminosity of only about 100,000 Suns.

Because of its distance it appears almost invisible to most telescopes. The previous dimness record holder was discovered last year in Ursa Major using SDSS-II data.

Searches for ultra-faint dwarfs are important because of a long-standing conflict between theory and observations. The leading theory of galaxy formation predicts that hundreds of clumps of cold dark matter should be orbiting the Milky Way, each one massive enough in principle to host a visible dwarf galaxy - but only 10 dwarf companions have been found to date.

One possibility is the galaxies in the smaller dark matter clumps are too faint to have appeared in previous searches, but might be detectable in deep surveys such as SDSS-II.

"It's like panning for gold. Our view of the sky is enormous, and we're looking for very small clumps of stars," said SDSS-II team member Wyn Evans of Cambridge.

"Finding and studying these small galaxies is really important," said team member Mark Wilkinson. “From their structure and their motions, we can learn about the properties of dark matter, as well as measure the mass and the gravity field of the Milky Way".

The new discoveries are part of the Sloan Extension for Galactic Understanding and Exploration project, one of the three component surveys of SDSS-II. SEGUE will probe the structure and stellar make-up of the Milky Way Galaxy in unprecedented detail.

"I'm confident there are more dwarf galaxies out there and SEGUE will find them," said Heidi Newberg of Rensselaer Polytechnic Institute in Troy, N.Y., co-chair of SEGUE.

Related Links
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ESO Detects Most Distant Hydrogen Cloud
Garching, Germany (SPX) May 10, 2006
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