Distinguished Lecture

Baryons and Dark Matter in the Galaxy

Professor Chris McKee
Department of Astronomy, University of California, Berkeley

Dark matter—matter that gravitates but does not radiate—has been studied by astronomers for more than a century. Evidence for its existence comes from observations of clusters of galaxies, of the rotation curves of galaxies and of the cosmic microwave background. Its nature is unknown; we do know that it is not baryonic and that its observational effects cannot be fully explained by modifying the law of gravity. The amount of dark matter in the solar neighborhood can be inferred by comparing the baryonic mass in stars and gas with the gravitating mass inferred from stellar kinematics. A new census of the baryonic mass near the Sun gives an improved estimate for the amount of local dark matter. Remarkably, the total density of matter near the Sun is within 1% of the value estimated by Oort in 1932! It has been suggested that there is a thin disk of dark matter in order to account for terrestrial phenomena that have a periodicity of about 30 Myr, but such a disk is difficult to accommodate. The Galaxy extends out to a distance of order 250 kpc, and observations indicate that this is filled with gas with temperatures in the range a few hundred thousand to a few million degrees. Models of the gas that fit the observations show that there is enough gas that there may be no “missing baryon” problem in the Galaxy, that it contains a significant amount of heavy elements and that there is enough of it to enable the Galaxy to continue to produce stars at the current rate for tens of billions of years.

Christopher McKee has carried out theoretical investigations of a wide variety of astrophysical phenomena, ranging from the interstellar medium of the Galaxy to quasars and cosmic gamma-ray bursts. His current research focuses on the formation of stars: How do low mass stars like the Sun form? How do the massive stars that create most of the heavy elements form? What determines the rate of star formation in galaxies? How did the first stars form? McKee received his AB degree from Harvard and his PhD in physics from UC Berkeley. After a year as a postdoctoral fellow at Caltech and several years as an assistant professor of Astronomy at Harvard, he joined the Physics and Astronomy departments at UC Berkeley, where he has been since 1974. He has received a number of honors for his work: He is a member of the National Academy of Sciences, a fellow of the American Academy of Arts and Sciences, of the American Association for the Advancement of Science, and of the American Physical Society. He has been the Sackler Lecturer at Harvard, at the University of Toronto, and at Tel Aviv University, the Bahcall Lecturer at Tel Aviv University, and the Antoinette de Vaucouleurs Medalist and Lecturer at the University of Texas. In 2016 he received the Henry Norris Russell Award for lifetime achievement from the American Astronomical Society.

Date & time

Thu 4 May 2017, 11am–12pm





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