Hopkins astronomers have seen evidence that a huge galaxy 50 million light-years from Earth is stealing clusters of stars from smaller neighboring galaxies, apparently in the act of merging with the satellites.
The team of astronomers, led by graduate student Eric Neilsen, used data from the Hubble Space Telescope to learn the distances to several galaxies, determine which of them are satellites of the much larger galaxy and then examine sizable groups of stars--called globular clusters--in each galaxy.
Inside the large galaxy, the astronomers found clusters of stars that likely formed in the satellite galaxies; the discovery suggests that large galaxies can have a significant effect on smaller neighbors, not only on the general distribution of individual stars but also on entire clusters of stars associated with those neighbors, said Neilsen, a doctoral student in the Department of Physics and Astronomy.
The findings were detailed in a paper written by Neilsen and his advisers, Hopkins astronomers Zlatan Tsvetanov and Holland Ford. The astronomers presented their findings on Jan. 7 during a meeting of the American Astronomical Society in Washington, D.C.
The research relates to the evolution of elliptical galaxies; they are roughly spherical, compared to another common type of galaxy, such as the Milky Way, which has a disk shape. Using data from the Hubble telescope's Wide Field Planetary Camera 2, the astronomers observed satellite galaxies in orbit around a giant elliptical galaxy called M87, which is seen in the constellation Virgo.
The observations revealed that M87 apparently contains star clusters that originally were from two neighboring galaxies, which are named NGC4486B and NGC4478. Neilsen specifically studied the history of the smaller satellite galaxies. Previous work has shown that these galaxies have compact shapes, and their outer parts seem to have disappeared.
"One model for why this has happened is that these parts have been stripped by a neighbor," said Neilsen, 27, who expects to complete the doctoral program in August.
The satellite galaxies themselves contain fewer star clusters than they should in their outer regions; the frequency of clusters falls off dramatically with increasing distance from the centers of the satellite galaxies, suggesting that the outermost clusters were stolen by M87.
The research confirmed previous findings that the star clusters come in two distinct colors, informally referred to as "red" and "blue." Furthermore, they revealed that the red clusters are found primarily near the center, while the blue clusters are seen both in the center and in the outer reaches of M87. That distribution of star clusters supports a theory that M87 formed through the merger of smaller galaxies; the theory predicts that most of the red clusters should be located near the center of M87, while the blue ones should still be found in the outer sections of the galaxy as well.
In 1994, Ford and Tsvetanov were among astronomers who used the Hubble telescope to find strong evidence for the presence of a massive black hole in the center of M87.
Neilsen came to Hopkins after completing undergraduate studies in 1992 at Kenyon College, in Gambier, Ohio, where he majored in physics. He plans to specialize in the astrophysics of elliptical galaxies and groups and clusters of galaxies.
An image of M87 and its companion galaxies can be viewed on- line at the following Internet address: