Like hunters tracking their prey through the forest, astronomers are retracing the movements of galaxies by following what they leave behind—a skill that has solved a 40-year-old cosmological puzzle. In the 1960s, radio telescopes revealed hundreds of gas clouds zipping around the Milky Way at unusual speeds. Some scientists later theorized that these "high-velocity clouds" are large clumps of hydrogen and dark matter that never gathered together to form a galaxy, relics from the early days of the universe. Mary Putman, an astrophysicist at the University of Colorado, finds just the opposite. The clouds are flecks of debris shed recently from small, nearby galaxies, she says.
Using the Parkes radio telescope in Australia, Putman and her colleagues originally set out to map the Magellanic Stream, a flow of hydrogen ripped from the Milky Way's two major satellite galaxies. Along the way, the researchers found that high-velocity clouds have many of ...
