Solving a Dark Puzzle


Animation: Click the image to watch this animation of how a galaxy like the Milky Way is built up according to the cold dark matter model, as well as how many dwarf galaxies (the small bright dots) there should be surrounding the Milky Way. Courtesy of J. Diemand, M. Kuhlen, P. Madau, Zemp, Moore, Potter and Stadel (UCSC). Send your comments on this issue of Cosmic Matters to .(JavaScript must be enabled to view this email address).

Image: This dwarf galaxy known as Leo II, seen in the Sloan Digital Sky Survey (SDSS), is one of the brighter dwarf galaxies near the Milky Way. The newly discovered dwarfs recently studied at Keck are up to 100 times fainter than Leo II. Courtesy of SDSS.

Since the first light of the Keck telescopes in 1992 and 1996 respectively, our understanding of the cosmos has grown enormously. One of the largest and still unexplained mysteries remains, however: dark matter. “Ordinary” matter, which we learned about in chemistry class, is made up of atoms and molecules and their constituent parts—- protons, neutrons, electrons, and various other subatomic particles that high energy physicists have discovered over the past 70 years.

Photo: Observations made by Keck astronomer Josh Simon are being used to further understand the nature of dark matter. Credit: S. Anderson/WMKO.

But in total, this regular matter makes up only about 15 percent of the matter in our Universe. The other 85 percent is dark matter, a mysterious substance that has not yet been detected directly because it does not give off any light. Astronomers first recognized its existence about 30 years ago by detecting its gravitational pull on things that they could see, such as stars and gas. According to the best current cosmological models, dark matter plays a crucial role in the early universe in pulling together ordinary matter to form the first galaxies, and even today remains the dominant component of galaxies like our Milky Way

Recent measurements of stars’ velocities using the Keck II telescope and its leading wide field multi-object spectrograph, DEIMOS, have provided new clues to the behavior of dark matter in nearby “dwarf” galaxies. These observations by Caltech astronomer Josh Simon and his Yale colleague Marla Geha have demonstrated that at least twice as many dwarf galaxies orbit around the Milky Way as was previously recognized, and are reshaping our understanding of the link between our Galaxy and its smallest neighbors.

Listen as Dr. Simon describes the history of dark matter research.

back to index