Mapping the Milky Way Halo

Robert Zinn

The halo of the Milky Way Galaxy is the roughly spherical region extending to more than 300,000 light-years from the Galactic Center. The halo was once thought to consist of ~ 80 globular clusters and a low density of very old stars that declined smoothly with distance from the Galactic Center. Recent surveys have revealed, however, a wealth of substructure that is probably debris from dwarf galaxies that have been torn apart by the tides produced by the gravitational force of the Milky Way. The diagram shows to scale the results of the first band of the QUEST survey for RR Lyrae variables in the halo (A. K. Vivas, Ph.D. thesis). The points show the locations of the variables that were detected in a fan-shaped survey of 380 sq. deg. of the sky that extends from ~12,000 to ~200,000 light-years from the Sun. At the edge of the survey is a large clump of RR Lyrae variables that belong to the northern stream of stars from the Sagittarius dwarf spheroidal galaxy, which is in the process of disintegration. This survey revealed for the first time a more tenuous substructure, the Virgo Stream, which has been shown by radial velocity measurements (S. Duffau, Ph.D. thesis) to consist of stars moving together through space. The QUEST RR Lyrae survey and the follow-up spectroscopy to measure the radial velocities and metallicities of the stars are collaborative projects involving astronomers at Yale, the Centro de Investigaciones de Astronomia in Venezuela, and the Departamento de Astronomia, Universidad de Chile.