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(E) Dr. Ivezic in The New York Times
By Nenad N. Bach | Published  10/31/2003 | Science | Unrated
(E) Dr. Ivezic in The New York Times


Croatian Scientist Dr. Zeljko Ivezic, an astrophysicist at Princeton University.

Halo Reveals Remains of Milky Way's Galactic Snacks


Astronomers have long suspected a dark secret about Earth's galaxy, the Milky Way. They have even seen other galaxies engaging in the macabre behavior at the core of this secret. But without irrefutable proof, astronomers refused to admit that it might be commonplace closer to home. Now, they have that proof.

The Milky Way is a cannibal. It consumes other galaxies and makes their stars its own.

The evidence comes from some of the first data to emerge from the multi-institutional collaboration called the Sloan Digital Sky Survey, an $80 million project that will eventually observe and categorize hundreds of millions of celestial objects over a quarter of the entire sky. Faint stars that the Sloan collected in two patches of the sky have revealed, in effect, the partly digested remains of two small galaxies that the Milky Way's gravity grabbed, perhaps a billion or two billion years ago.

The clumpy patches turned up not in the bright disk of the galaxy, but in the thinly populated sphere of stars and other matter around it. The telltale remains hover about 150,000 light-years from the galactic center.

"It looks like the stars are formed in other galaxies, and then are being assimilated into our galaxy," said Dr. Heidi Newberg, an astrophysicist at Rensselaer Polytechnic Institute in Troy, N.Y., and a Sloan collaborator. The inevitable conclusion, she said, is that "we're munching on the little ones," or eating up smaller galaxies that once glittered on the outskirts of the Milky Way.

Dr. Newberg said it was still uncertain whether all of the stars of the halo were captured in this way. But the new results do provide strong support for theories holding that the halo was assembled bit by bit after the disk formed. Alternative theories maintain that the halo formed all at once, and that material inside it later settled into a disk.

The data were assembled from observations of two patches of sky, one in the northern galactic plane and one in the south, made by the Sloan Survey's automated telescope at Apache Point, N.M., said Dr. Brian Yanny, an astrophysicist at the Fermi National Accelerator Laboratory and a Sloan collaborator.

The telescopic images included a few million stars all told. But within that sample, the images revealed elongated clumps of hot objects called A stars. The northern clump contained about 2,000 A stars and the southern clump about 1,000.

The northern clump also turned out to be traced by another kind of star, the blinking variable stars called RR Lyraes, said Dr. Zeljko Ivezic, an astrophysicist who is a Sloan collaborator at Princeton University.

Both types of star have a special property: Their intrinsic brightness is approximately known, so their distance from Earth can be estimated from their apparent brightness on the sky. That knowledge allowed the Sloan astronomers to reconstruct the three-dimensional shapes of the clumps.

"They're very diffuse and elongated," Dr. Yanny said, or roughly cigar shaped, 15,000 light-years wide and 60,000 light-years long. They no longer look like little galaxies, he said, because "they've been disrupted by the gravitational potential" of the Milky Way -- partly digested, in less clinical language.

Such faint groupings of stars can be seen only using a vast survey like the Sloan, Dr. Ivezic said. He added that the precise way in which the former galaxies were being torn to pieces could reveal the distribution of a mysterious, nonluminous type of matter called dark matter in the halo. "That will constrain very strongly how the halo is built," Dr. Ivezic said.

The finding supports the basic theoretical picture of large structures in the universe being formed from smaller ones, said Dr. Abraham Loeb, an astrophysicist at Harvard University, who is not a member of the Sloan Survey.

And as for the Milky Way, Dr. Loeb said, "you can call it a cannibal. We have to eat to feed ourselves. And a galaxy feeds on these smaller components."


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