Deep Impact from Extragalactic Space: Debris from a Shattered Galaxy discovered in the Sun's Backyard
Nearby relics of a long-dead galaxy have been unearthed by astronomers from Leiden Observatory and from the Max-Planck-Institute for Astrophysics using data from the European Space Agency's Hipparcos Satellite (Nature, 4 November 1999).
The comet debris which apparently killed off the dinosaurs survives as a thin layer of iridium-rich dust covering their fossilised bones. In the same way, debris from a dwarf galaxy destroyed by a collision with our own Galaxy can survive as relic streams of stars wrapping around the Milky Way. What should such streams look like and how can they be detected? Computer simulations of galaxy collisions offer some answers. Even galaxies destroyed during the Milky Way's infancy should still be detectable as ghostly sheets of stars moving on near-parallel orbits. These theoretical results encouraged the astronomers to search for real relic streams, and led to the discovery of two nearby streams which appear to be debris from a single dwarf galaxy torn to shreds by the Milky Way's gravity about 10 billion years ago.
Between 1989 and 1993 the European Space Agency's Hipparcos satellite measured the positions, distances and motions of hundreds of thousands of stars. The resulting catalogues, published in 1997, provide a unique opportunity to search for stellar streams. By sifting carefully through these catalogues it was possible to make a nearly complete census of old giant stars within a few thousand light years of the Sun. (For comparison, our Milky Way galaxy as a whole is about 50,000 light years across.) Earlier observations had used mountain-top telescopes to show that these stars contain less than a thirtieth as much iron and magnesium as the Sun, a sure sign that they are among the oldest stars in the Milky Way. These observations also measured how fast each star moves away from the Sun. Hipparcos, on the other hand, measured changes in the apparent direction of each star, thus establishing how fast it moves across the line-of-sight. Combining these data determined the direction of motion and the speed of a few hundred nearby giant stars.
|Figure: The top picture shows an image of the real sky made by the DIRBE instrument on NASA's Cosmic Background Explorer satellite COBE. At the wavelengths chosen to make this picture our own Galaxy, the Milky Way, shows up beautifully and looks very much like a distant galaxy seen edge-on. The middle picture shows a computer simulation of the disruption of a dwarf galaxy set up to resemble the recently discovered disrupting galaxy in Sagittarius. Stars from the dwarf galaxy are seen all around the sky but are confined to a narrow band near the galaxy's orbit. The black region shows the remaining body of the dwarf. Finally, the bottom picture shows a computer simulation of the debris from a dwarf that was destroyed during the Milky Way's infancy. This simulation was designed to produce streams similar to those discovered by the Leiden-Garching team. Stars are seen all over the sky with no remaining sharp features. The structure of the system only becomes clear when the motions and distances of the stars are measured. They all lie on sheets wrapped many times around the Milky Way, with the stars near any particular point on a sheet all moving on nearly parallel orbits.|
Analysing these observations, the Leiden-Garching team was struck by the fact that while most stars move in apparently random directions, two small sets of stars seem to be moving together. One set of nine stars pursue near-parallel orbits which cross the Milky Way's disk at high speed from North to South; a second set of three stars pursue orbits which cross the disk at the same speed and angle, but from South to North. In both cases the direction and speed of the motion is well separated from those of the other nearby giants. Comparing with their published computer simulations, the team concluded that each set of stars is part of a debris stream, and that both streams come from a single dwarf galaxy which was torn apart by the Milky Way's gravity during or very soon after the Milky Way's own formation. The characteristics implied for this dwarf make it very similar to other small galaxies which still survive in the outskirts of the Milky Way system, as well as to the recently discovered Sagittarius Dwarf Galaxy which is just now being shredded by the Milky Way. The stars in each set are distributed all over the sky, even though they move in parallel, showing that the Sun is actually sitting inside the two streams of debris.
The deeper significance of this discovery may lie in its implications for the Milky Way's formation. Our Galaxy has two major components. One is a highly flattened disk in which gas and stars circle the Galactic centre rather as the planets circle the Sun. The other is almost spherical and consists purely of old stars moving in near-random directions like bees in a swarm. The giants now studied are nearby members of this second population. Most astronomers agree that the disk grew slowly as gas settled onto circular orbits and turned into stars. There is no consensus, however, about how the older component was assembled. Yesterday's wisdom, that it formed by the rapid collapse of a single massive gas cloud, is disputed by recent theories which suggest that it grew through the amalgamation of pre-existing smaller galaxies. In recent years galaxies have been discovered at such enormous distances that they are seen, not as they would be today, but as they were when they were young. These distant galaxies are indeed much smaller than older and nearer systems. They could, perhaps, be the building blocks of the near-spherical component of the Milky Way. The ghostly streams discovered by the Leiden-Garching team may be the first fossil evidence showing directly that our own Galaxy was put together from such pieces.
Text from the Press release of the MPG