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A bigger Andromeda galaxy.

BEING THE CLOSEST large galaxy to our Milky Way, the Andromeda Galaxy (M31) has been one of the most intensely studied objects in the history of astronomy. But several new discoveries show that it still has surprises to offer.

After taking deep images and follow-up spectroscopic data at the 10-meter Keck II Telescope in Hawaii, a team led by Rodrigo Ibata (Strasbourg Observatory, France) identified stars belonging to a faint disk that can be followed to an astounding 230,000 light-years from M31's nucleus. That's 9 percent of the way to Earth. This structure, which might account for 10 percent of the disk's total luminosity, indicates that at least some galaxies' stellar disks extend much farther out than previously thought. This new study builds upon results from two other teams who also concluded that M31 has an extended stellar disk.

Ibata and his colleagues suggest in an upcoming Astrophysical Journal paper that the disk formed from the accretion of one or more dwarf galaxies, possibly including stars from M31's satellite galaxy M110 (NGC 205). "We show that the disk assimilates small galaxies or galaxy fragments, so it is forming by star accretion in its outer edge," says Ibata. "This is quite unexpected. Some substantial revision of the current ideas of disk formation is needed."

In an upcoming Nature paper, Puragra Guhathakurta (University of California, Santa Cruz) and his colleagues report the first identification of isolated stars in M31's extended halo. Previously, astronomers had spotted M31 extended-halo stars only in globular clusters. Using observations from the 4-meter Mayall Telescope on Kitt Peak and the Keck II Telescope, the team developed an efficient technique to identify red giants in M31's halo against the dense veil of Milky Way foreground stars. The team traced M31 halo stars to distances of at least 500,000 light-years from the galaxy's center. Their velocities and heavy-element compositions suggest that they also came from cannibalized dwarf galaxies.

"Our result shows that the Milky Way is not alone in having an extended halo," says team member Steven R. Majewski (University of Virginia). "Astronomers have postulated that all galaxies are embedded in dark halos, but in M31 we now have proof that another spiral galaxy also has a stellar halo like that of our own galaxy."

An Italian team led by Flavio Fusi Pecci (Bologna Observatory) has discovered several massive globular clusters embedded in M31's disk at a distance of about 65,000 light-years from the core. Surprisingly, these clusters appear to be less than 2 billion years old, much younger than all known Milky Way globulars (see the facing page). This implies a bout of vigorous star formation in M31's disk 2 billion years ago.

An international team led by Avon P. Huxor (University of Hertfordshire, UK) has found three peculiar clusters of old stars near M31 that have sizes and concentrations between those of dwarf galaxies and globular clusters. "These clusters could be the remnants of dwarf galaxies that have been tidally shredded by M31," says Geraint F. Lewis (University of Sydney, Australia), a member of both the Huxor and Ibata teams. "But we are left with the problem of why we don't see any of these things around the Milky Way. Perhaps M31 has had a more violent recent history consuming dwarfs, and these fuzzy blobs are some of the leftovers."

The discovery of these objects, along with the similar but lower-luminosity cluster Willman 1 found near the Milky Way last year, continues to blur the distinction between dwarf galaxies and globular clusters. The globular M54, for example, appears to be the core of the Sagittarius dwarf galaxy that is being tidally shredded by the Milky Way (July issue, page 16). Omega Centauri (NGC 5139) could also be the core of a tidally stripped dwarf galaxy. The newly discovered M31 clusters could be smaller versions of Omega Centauri.

The Milky Way and M31 have similar masses, but M31 is slightly more luminous, has a bigger bulge and disk, and has three times as many globular clusters. The differences between the Milky Way and M31 appear to be largely the result of the random manner in which they accreted surrounding dwarf galaxies.

Astronomers have also turned their attention to M31's core. After scrutinizing archived Chandra X-ray Observatory data, a team led by Michael R. Garcia (Harvard-Smithsonian Center for Astrophysics) has probably detected X-ray emission from M31's supermassive black hole, known as M31*. The black hole has a mass of about 30 million Suns, but it appears much dimmer in X-rays than expected, meaning it accretes matter extremely inefficiently.
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Author:Naeye, Robert
Publication:Sky & Telescope
Geographic Code:1USA
Date:Aug 1, 2005
Words:761
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