Swallow Thy Neighbor: Strong evidence of galactic cannibalism

It’s a violent world out there, and many large galaxies have the corpses to prove it. These massive galaxies, including our own Milky Way, are surrounded by streams of gas and stars, the fossil remains of dwarf galaxies that they tore apart long ago (SN: 7/7/01, p. 5: Available to subscribers at Andromeda feasts on its satellite galaxies). But examples of dwarf galaxies still in the process of giving up their material to a larger partner have proved more elusive, even though the standard theory of galaxy assembly suggests that such cannibalism is common.

RIPPER. Hubble image shows plumes of stars (purple in inset) being cannibalized by a large spiral galaxy (large white area in inset and arrow in main image). Tadpole galaxy is upper-left in main image. The spiral galaxy and its dwarf satellite lie about 2 billion light-years from Earth. NASA/STScI

Astronomers say that they now have a compelling case of a big galaxy caught in the act of eating a small fry. Some 2 billion light-years from Earth, a galaxy about as large as the Milky Way is pulling two plumes of stars from a tiny satellite galaxy, report Duncan A. Forbes of the Swinburne University of Technology in Hawthorn, Australia, and his colleagues in the Aug. 29 Science. The findings are further evidence that galaxies start out small and grow bigger over time, in part by consuming their smaller brethren, Forbes says.

The discovery appears to be “the most spectacular example of such a satellite disruption seen so far outside the Local Group of galaxies,” comments Franois Schweizer of the Carnegie Observatories in Pasadena, Calif.

Study coauthor Michael A. Beasley of Swinburne made the initial discovery when he examined one of the first images taken by the Hubble Space Telescope’s ultrasharp Advanced Camera for Surveys. The image, recorded in April 2002, features an eye-catching view of a spiral galaxy called the Tadpole, but Beasley was drawn to one of the thousands of unnamed galaxies in the background. This body, also a spiral galaxy, appears to be adjacent to two plumes of stars, each originating from a small blob–perhaps a satellite galaxy.

Spectra of the background galaxy and the blob taken with the Keck 1 Telescope atop Hawaii’s Mauna Kea then revealed that the two bodies reside at the same distance from Earth and therefore are physically connected.

Computer simulations show that the presence of the two plumes and their orientations are just what would be expected if the blob is a dwarf galaxy and its stars are being ripped away by its neighbor.

Forbes cites several reasons that it’s been hard to find satellite galaxies just beginning to be torn asunder. The dwarf galaxies and their plumes are extremely faint, he notes. Moreover, most tiny galaxies seem to have highly elongated orbits and to spend much time far from their larger, more luminous partners, so the pairs are difficult to pick out.

Even so, the new findings don’t solve a discrepancy within the leading theory of galaxy formation. That theory holds that an invisible type of material known as dark matter makes up most of the mass in the universe. Its structure indicates that there ought to be 100 to 1,000 times more small galaxies orbiting bigger ones than astronomers have observed (SN: 10/13/01, p. 234: A Cosmic Crisis?).

With recently refined galaxy simulations and the large, high-resolution telescopes currently available, the tools are in hand to conduct a census of such galaxies, says Kathryn V. Johnston of Wesleyan University in Middletown, Conn.


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