MILKY WAY'S MOTION REVEALED THROUGH DISTANT EXPLODING STARS
CAMBRIDGE, Mass.--Our own Milky Way galaxy and its nearby neighbors are streaming in the direction of the constellation Leo, report astronomers at the Harvard-Smithsonian Center for Astrophysics. This conclusion--gleaned from observations of exploding stars known as supernovae--confirms the direction found by mapping the radio emission from the Big Bang. The new finding differs, however, from recent results obtained from measurements of distant clusters of galaxies.
In the past decade, astronomers have noted that some clumps of galaxies, like the Milky Way and about two dozen surrounding galaxies (known as the "Local Group") are moving toward presumed distant concentrations of mass at a speed and direction differing from a simple, uniform expansion of the Universe.
"Understanding how these gangs of galaxies flow through space gives us a better idea of how mass and light are distributed in the Universe," says astronomer Robert Kirshner of the Harvard- Smithsonian CfA, who led the study.
Analyzing data on 13 supernovae gathered by astronomers at Cerro Tololo International Observatory in Chile and at the Whipple Observatory in Arizona, Kirshner and his team plotted the supernova's brightness as it faded over time. The way supernovae brighten and then fade enables astronomers to gauge the distances to these distant, exploding stars more accurately. This new distance-determining procedure, known as the light curveshape, or LCS method, was developed by Kirshner's graduate student, Adam Riess, in collaboration with study co-author William Press.
The researchers also knew the redshifts of each of the 13 galaxies that was host to each supernova--ranging from 75 to 150 million light years away. A redshift value tells astronomers how fast a cosmic object is moving away from Earth. In a smoothly expanding Universe, the redshift is proportional to the distance, as discovered by Edwin Hubble in 1929. However, the presence of currents that depart from a smooth flow can be detected by combining redshift information with the distances from the LCS technique.
Armed with the distances from the supernovae, redshifts for the galaxies in which the supernovae occurred, and the distances the galaxies would have, if there were no currents among the galaxies, the scientists were able to discern a pattern of apparent motion of the distant galaxies that reflect the motion of our own Milky Way galaxy. "It's like being in a boat on a lake," explains Kirshner. "You can figure out your own motion by noticing one shore receding and the other shore approaching."
The direction they detected is similar to that found in earlier studies of the cosmic background radiation--the faint hiss of microwave radiation emanating uniformly from all directions in the sky, commonly referred to as the "echo of the Big Bang." A subtle pattern of variation in the temperature of the background radiation reflects the Milky Way's motion against this more distant background.
This result contrasts with recent reports by Todd Lauer of Kitt Peak National Observatory and Marc Postman of Space Telescope Science Institute based on measurements of distant galaxy clusters which suggested that the Milky Way and a large region of surrounding galaxies are moving rapidly, when compared to the microwave background. The new supernova-based results suggest, however, that the motions of galaxies on large scales are smaller, and maybe accounted for within current pictures of the way matter is clumped on large scales.
One limitation of this motion-measuring technique, notesKirshner, is that supernovae areunpredictable--astronomers don't know when or in whichgalaxy they will occur. Nonetheless, the 13 used in this study are fairly well-distributed in the sky. All of thesupernovae were Type Ia, thought to originate in thesudden thermonuclear explosions of white dwarf stars. Eachyear, about 50 new supernovae are detected by astronomersworldwide. Between 10 to 20 of these may proveappropriate targets for obtaining light curve shape data, Kirshner anticipates. Such information could confirm the motion of the Local Group that he and colleagues observed. They reported their findings in the June 1, 1995 issue of The Astrophysical Journal Letters.
The ultimate goal, says Kirshner, is to map the motions of galaxies out to distances of 300 million light years. Since three-dimensional maps of the galaxy distribution at these distances are underway, the combined information on the lumpiness of galaxies and the currents induced in the galaxy motions will lead to a better understanding of the unseen mass--or dark matter--that seems to dominate the dynamics of the Universe.