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Galaxy Disruptions Probe Dark Matter
June 7, 2006
 Astrophysicists announced this week that the tidal disruption of smaller galaxies by our own Milky Way could be the most sensitive probe yet of long-range forces affecting dark matter. The report is being presented at the 208th meeting of the American Astronomical Society (AAS) held in Calgary, Alberta, by Michael H. Kesden of the Canadian Institute for Theoretical Astrophysics (CITA) at the University of Toronto in Toronto, Ontario. The work was begun in collaboration with Marc Kamionkowski, professor of theoretical physics and astrophysics, while Kesden was a graduate student at Caltech. This result is of special interest because the detection of such long-range forces could help to finally identify the origin of the mysterious dark matter, one of the great unsolved problems of modern physics.Astronomers cannot observe dark matter directly, but they can detect its existence through its gravitational influence on the stars and gas in galaxies and galactic clusters. Recent surveys of the cosmic microwave background (CMB) and the large-scale structure of the universe have confirmed that there is about five times as much dark matter in the universe as there is normal matter in the form of stars and gas. Yet other than the total mass of dark matter, and the fact that it is moving much slower than the speed of light, we know practically nothing about this huge component of our universe. Even more surprising than dark matter, astronomers have discovered that the expansion of the universe is accelerating due to an unexplained dark energy spread uniformly throughout the universe. In many theories this dark energy interacts with both visible and dark matter, leading to a "fifth force" in addition to gravity, electromagnetism, and the strong and weak nuclear forces. Laboratory experiments have placed tight constraints on any fifth force affecting visible matter, but these limits don't apply to the dark matter. How can we hope to measure forces affecting dark matter when we can't even detect the dark matter itself directly? One possible way of constraining dark-matter forces involves studying their effect on the tidal disruption of satellite galaxies by a larger host. According to current theory, galaxies like our own Milky Way form hierarchically through the merger of smaller structures. This process continues to the present day, as several satellite galaxies have been observed that may one day merge into the larger Milky Way halo. Just as the moon and sun raise tides on Earth's oceans, so too does the Milky Way raise tides on the stars and gas of these satellite galaxies, in some cases pulling them free entirely. Disrupted stars either trail or lead the satellite galaxy in its orbit around the Milky Way, depending on whether they have gained or lost energy through the tidal interaction. These trailing and leading tidal streams are an ideal laboratory for studying dark-matter forces. The basic idea behind using tidal streams to constrain dark-matter forces is elegant in its simplicity. While the satellite galaxy remains dominated by dark matter and responds to forces from the Milky Way's dark-matter halo, the disrupted stars are free to orbit the Milky Way under the influence of gravity alone. An attractive dark-matter force will pull the satellite around its orbit faster than gravity alone, leading to a relative enhancement in the trailing compared to the leading tidal stream. A repulsive dark-matter force will conversely slow the satellite down causing the disrupted stars to lead ahead in its orbit. By comparing the observed ratio of leading-to-trailing stars to that found in simulations, one might hope to detect a dark-matter force even if only 1 percent the strength of gravity. "What we're doing here is a 21st-century equivalent of Galileo's leaning-tower experiment. Galileo demonstrated that terrestrial materials all fall in the same way in a gravitational field, and we're trying to figure out whether his conclusion applies to dark matter as well," says Dr. Kamionkowski. An ideal satellite in which to search for dark-matter forces is the Sagittarius (Sgr) dwarf spheroidal galaxy, located approximately 78,000 light-years from Earth on the opposite side of the Milky Way. Detailed observations of the Sgr dwarf and its tidal streams have been undertaken by both the Two Micron All-Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS). 2MASS observations were performed using 1.3-meter (50-inch) telescopes at the Whipple Observatory on Mt. Hopkins, Arizona, and the Cerro Tololo Inter-American Observatory in Chile, while SDSS observations made use of a dedicated 2.5-meter (100-inch) telescope at the Apache Point Observatory in the Sacramento Mountains of New Mexico. Both surveys were funded by the National Aeronautics and Space Administration (NASA) and the National Science Foundation, among other institutions. While the simulations presented by Kesden at the Calgary meeting of the AAS and the Canadian Astronomical Society suggest that dark-matter forces with a strength a few percent that of gravity could be detected in the Sgr dwarf, it is only through detailed comparison with these and future observations that such an effect could be conclusively demonstrated. For more information, contact Michael H. Kesden (416-978-1777, kesden@cita.utoronto.ca), or Dr. Marc Kamionkowski (626-395-2563, kamion@tapir.caltech.edu).
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