When Vera Rubin arrived in 1965, the Department of Terrestrial Magnetism (DTM) was a hands-on physics laboratory, and Kent Ford, a young Staff Member, had just designed and built an image tube spectrograph. This state- of-the-art instrument allowed telescopes to observe objects that were many times fainter than those that had previously been studied. Rubin’s interest in how stars orbit their galactic centers led her and Ford to study the nearby spiral M31, the Andromeda galaxy. The two researchers hoped to determine the distribution of mass in M31 from the orbital speeds of stars and gas at different distances from the galaxy’s center. Newtonian gravitational theory states that an object farther from its central mass will orbit slower. But, to their surprise, the scientists found that stars far from the center traveled as fast as those near the center.

By the late 1970s, after Rubin and her colleagues had observed dozens of spirals, it was clear that something other than the visible mass was responsible for the stars’ motions. Analysis showed that each spiral galaxy is embedded in a spheroidal distribution of dark matter — a “halo.” The matter is not luminous, it extends beyond the optical galaxy, and it contains 5 to 10 times as much mass as the luminous galaxy. The stars’ response to the gravitational attraction of the matter produces the high velocities. As a result of Rubin’s groundbreaking work, it has become apparent that more than 90% of the universe is composed of dark matter. Defining it is one of astronomy’s most important pursuits.

During the 1970s, Rubin and DTM collaborators Ford, Norbert Thonnard, and John Graham were among the first astronomers to examine the systemic velocities of galaxies to see if there are large-scale motions of galaxies, superposed on the general expansion of the universe. Their early work, and more recent work by others, suggests that such motions exist. Accurate details of these motions require large data sets for thousands of galaxies. Several large astronomical consortia are now making extensive observations to address this question.

This is an image of the Andromeda galaxy (M31), the companion spiral to our own, copied from the Palomar Sky Survey. The measured optical velocities from ionized gas clouds are indicated as open and filled circles. Velocities from neutral hydrogen radio observations are shown as filled triangles. Note that the velocities remain high far beyond the optical disk.

(Montage by Vera Rubin and Janice Dunlap, based on Roberts.)

Recently Rubin has been observing low-surfacebrightness galaxies, objects that are fainter than the night sky. In these galaxies, the stars contribute little to the total mass; most of the mass is composed of dark matter. Because the inner rise of the rotation curve will differ depending upon the properties of the dark matter, Rubin and colleagues are using their observations to attempt to discriminate between various models for the composition of the dark halos.

SELECTED PUBLICATIONS

• Swaters R. A., and V. C. Rubin. 2003. Stellar motions in the Polar Ring Galaxy NGC 4650A, Astrophys. J. Lett., 587, L23.
  
  
• Rubin, V. C. 2002. Why does an optical astronomer study something she cannot see? in Beyond Earth: Mapping the Universe, D. DeVorkin, ed., Washington, D.C., National Geographic Press, pp. 186-203.
  
  
• de Blok, W. J. G., S. McGaugh, A. Bosma, and V. C. Rubin. 2001. Mass density profiles of low surface brightness galaxies, Astrophys. J. Lett. 552, L23.
  
  
• Rubin, V. C., and J. Haltiwanger. 2001. Disturbed kinematics in Virgo Cluster spirals, in Galaxy Disks and Disk Galaxies, ASP Conference Series 230, J. G. Funes and E. M. Corsini, eds., pp. 421-424, Astronomical Society of the Pacific, San Francisco.
  
  
• Sofue, Y., and V. C. Rubin. 2001. Rotation curves of spiral galaxies, Annu. Rev.
  Astron. Astr. 39, 137.

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