Huge Galaxy May Steal Clusters Of Stars From Nearby Galaxies

Johns Hopkins University astronomers have seen evidence thata huge galaxy, about 50 million light years from Earth, removesclusters of stars from neighboring galaxies.

The team of astronomers, led by graduate student EricNeilsen, used data from the Hubble Space Telescope to learn thedistances to several galaxies, determine which of them aresatellites of the much larger galaxy and then examine groups ofstars -- called globular clusters -- in each galaxy.

Inside the large galaxy, the astronomers found clusters ofstars that likely formed in the satellite galaxies; the discoverysuggests that large galaxies can have a significant effect onsmaller neighbors, not only on the general distribution ofindividual stars but also on the clusters of stars associatedwith those neighbors, Neilsen said.

The astronomers presented their findings on Wednesday,Jan. 7, 1998, in a poster paper on display during a meeting ofthe American Astronomical Society in Washington, D.C.

The paper's authors were Neilsen and Johns Hopkinsastronomers Zlatan Tsvetanov and Holland Ford. Neilsen is a doctoral candidatein the Department of Physics and Astronomy. His advisors are Tsvetanov, aresearch scientist, and Ford, a professor, in the same department.

The research relates to the evolution of ellipticalgalaxies; they are roughly spherical, compared to another commontype of galaxy, such as the Milky Way, which has a disk shape.Using data from the Hubble telescope's Wide Field PlanetaryCamera 2, the astronomers observed satellite galaxies in orbitaround a giant elliptical galaxy called M87, which is seen in theconstellation Virgo.

The observations revealed that M87 apparently contains starclusters that originally were from two neighboring galaxies,which are named NGC4486B and NGC4478. The research specificallystudies the history of the smaller satellite galaxies. Previouswork has shown that these galaxies have compact shapes, and theirouter parts seem to have disappeared.

"One model for why this has happened is that these partshave been stripped by a neighbor," Neilsen said. "Our dataindicate that where the unclustered stars that make up most ofthe galaxy appear to have been stripped, the globular clustersalso appear to have been stripped."

The satellite galaxies themselves contain fewer starclusters than they should in their outer regions; the frequencyof clusters falls off dramatically with increasing distance fromthe centers of the satellite galaxies, suggesting that they werestripped away by M87.

The astronomers confirmed that the clusters of stars came intwo distinct colors, informally referred to as "red" and "blue."Furthermore, they discovered that the red clusters are found primarily nearthe center, while the blue clusters are seen both in the centerand in the outer reaches of M87. These findings support a theorythat M87 formed through the merger of smaller galaxies; thetheory predicts that most of the red clusters should be locatednear the center of M87, while the blue ones should still be foundin the outer sections of the galaxy as well.

In 1994, Ford and Tsvetanov were among astronomers who usedthe Hubble telescope to find strong evidence for the presence ofa massive black hole in the center of M87.

(Journalists may obtain an image of M87 and its companion galaxies from theWorld Wide Web at http://adcam.pha.jhu.edu/~neilsen/press/m87map.html. A hardcopy of the image also is available by calling Emil Venere at the JohnsHopkins Office of News and Information at 410-516-7160.)