Alan Dressler

One of the great mysteries of modern astronomy is the nature, amount, and distribution of the dark matter that dominates the universe. A principal direction of Alan Dressler's research has been to try to map the distribution of dark matter on cosmic scales by tracing its presence through the gravitational pull it exerts on galaxies. By finding the "peculiar velocities" of thousands of galaxies in our cosmic neighborhood, Dressler and his colleagues discovered a huge concentration of dark matter, which they named the Great Attractor.

This year marked a milestone in the study. Data from the new technique called surface brightness fluctuations have at last been brought to bear on the problem. Many of the new measurements were made with the du Pont CCD cameras and relied on the excellent observing conditions at Las Campanas Observatory. The data took almost 10 years to obtain, and provide the most accurate peculiar velocities to date. In three new papers, Dressler and his colleagues John Tonry of the University of Hawaii, John Blakeslee of Caltech, and Ed Ajhar of the National Optical Astronomy Observatories confirm the existence of the Great Attractor. Additionally, the researchers were able to see, for the first time, the weaker flows of galaxies into much smaller mass concentrations, as well as the flows away from voids -- places where few galaxies are seen and the corresponding mass density is thought to be very low. The higher resolution and sensitivity of these new dark-matter maps will allow a much better determination of the extent to which the galaxy distribution traces the dark matter. This information is crucial to interpreting the large-scale structure of the universe as mapped by galaxies, and to understanding the role of dark matter in galaxy formation.

Another theme of Dressler's research involves the question of how galaxies formed and evolved over cosmic time. Astronomers have the unique ability to look back in time as they look far out into space. With their colleagues, Dressler and Gus Oemler have been using large ground-based telescopes and the Hubble Space Telescope to observe distant galaxies to determine their properties at earlier times. This collaboration, known as the MORPHS Project, has produced two major papers this year. The papers describe the connection between galaxy morphology (structure) and star-formation history in galaxies seen as they were 5 billion years ago. Among the conclusions of this work is that bursts of star formation were much more common in galaxies at that epoch, in contrast to the steadier star-formation rate characteristic of galaxies today. Additionally, the scientists found that because these starbursts were often shrouded by dust, their importance had been underestimated in earlier work.

The MORPHS study concluded that elliptical galaxies -- the spheroidal systems that are most common in regions of high galaxy density -- are generally old, nearly as old as the universe, but that over the last 5 billion years many spiral galaxies -- disk galaxies with ongoing star formation -- have been transformed to a dormant kind of disk galaxy called an S0. The specific mechanisms responsible for this change remain elusive and are still under study by the group.

Over the next two years Dressler will devote considerable effort as principal investigator of the IMACS instrument, a sensitive multiobject spectrograph and camera with a very wide field. When mated with the new Magellan I telescope, this instrumentation will open new territory in the high-redshift universe for Dressler and colleagues to study the evolution of galaxies with lookback time.