Astronomers using the Canada-France-Hawaii and W. M. Keck Observatory telescopes on the summit of Mauna Kea, Hawaii, have been amazed to find a group of dwarf galaxies moving in unison in the vicinity of the Andromeda Galaxy. The structure of these small galaxies lies in a plane, analogous to the planets of the solar system. Unexpectedly, they orbit the much larger Andromeda Galaxy en masse, presenting a serious challenge to our ideas for the formation and evolution of all galaxies.
While Persian astronomers were the first to catalog the Andromeda Galaxy, it’s been only in the past five years that scientists have studied in detail the most distant suburbs of the Andromeda Galaxy via the Pan-Andromeda Archaeological Survey (PAndAS), undertaken with the Canada-France-Hawaii Telescope and measured with the Keck Observatory, providing our first panoramic view of our closest large companion in the cosmos.
The study culminates many years of effort by an international team of scientists who have discovered a large number of the satellite galaxies, developed new techniques to measure their distances, and have used the Keck Observatory with colleagues to measure their radial velocities, or Doppler shifts — the speed of the galaxy relative to the Sun. While earlier work had hinted at the existence of this structure, the new study has demonstrated its existence to a high level of statistical confidence — 99.998 percent.