An international team of astronomers using NASA's Hubble Space Telescope has discovered a system of star clusters which was created, or perhaps orphaned, during the destruction of its parent galaxy.
Hubble's superb resolution has enabled the identification of a class of small and very faint star clusters in the central region of NGC 1316, a giant elliptical galaxy in the southern constellation Fornax. Many of these clusters are so small that they are barely held together by the mutual gravity of their constituent stars.
Though such clusters are common in spiral galaxies like our Milky Way, they have rarely been seen in elliptical galaxies. The astronomers conclude that these clusters are among the last visible remains of a galaxy which was cannibalized by NGC 1316.
That NGC 1316 has swallowed whole galaxies is evident in the wispy, smoke-like tendrils of dust, which are believed to be the remains of a gas-rich galaxy that collided and merged with NGC 1316 sometime during the last hundred million years. Other signs of a violent past include arcs and plumes of stars emanating from the outskirts of the galaxy.
NGC 1316 is a tremendously powerful source of radio waves, which are thought to be generated by a massive black hole at the galaxy's center. The gaseous remains of the cannibalized galaxy are falling towards the center of NGC 1316, where they may ultimately end up feeding the black hole and providing the energy needed to sustain enormously energetic jets of material which extend outwards for more than 250,000 light-years.
The team, led by Carl Grillmair of the California Institute of Technology, has been using the Wide Field Planetary Camera 2 on the Hubble Space Telescope to carry out a systematic study of star clusters around relatively nearby elliptical galaxies. Their study, which has been published in The Astronomical Journal, focused on globular clusters, which are spherical swarms of stars with hundreds of thousands or even millions of members. The new results will shed light on the dynamics of galaxy collisions and mergers, which were more common in the universe long ago, and may have occurred quite frequently in the early days of the Milky Way.
Hubble's high resolution permits astronomers to study star clusters much fainter than those which can be detected by ground-based telescopes. By studying the brightnesses, colors, and distributions of these clusters within their parent galaxies, the team has improved our understanding of the ways in which galaxies form and evolve. Though giant elliptical galaxies are usually surrounded by thousands of globular clusters, the Hubble images of NGC 1316 show surprisingly few. Instead, the team found a large population of much fainter star clusters, containing only a few thousand stars each. Based on previous Hubble discoveries of very bright, very blue, and very young globular clusters in the centers of galaxies which have undergone recent mergers, Grillmair and his team were surprised to find only these older and relatively dim cousins.
"The star clusters are too old to have been born during the collision which produced the dusty debris we see today, but they have not been around long enough to have been torn apart by galactic tidal forces," says Grillmair. "This means that they must either have been created in the course of a still earlier collision, or that they belonged to a galaxy which fell victim to NGC 1316. By following up the Hubble images with spectroscopic observations, astronomers will be able to study in detail how galaxies are stripped of their stars and gas in the course of galactic mergers."
The team members are Carl Grillmair (SIRTF Science Center/California Institute of Technology), Duncan Forbes (University of Birmingham), Rebecca Elson (University of Cambridge), and Jean Brodie (University of California).
Space Telescope Science Institute, Baltimore, MD
California Institute of Technology, Pasadena, CA