I am involved primarily in observational studies at optical, ultraviolet, and near-infrared wavelengths. Most of my data are obtained with the Keck 10-meter telescopes, the Lick 3-meter reflector, and the Hubble Space Telescope, though I also utilize the Chandra X-ray Observatory and some other facilities. In addition, my group has developed a 0.8-meter robotic telescope at Lick Observatory (KAIT, the Katzman Automatic Imaging Telescope) that obtains data automatically, every clear night, while we sleep; see http://astron.berkeley.edu/~bait/kait.html.
My collaborators and I have made a concerted effort to determine the nature of the progenitor stars, the explosion mechanisms, and the nucleosynthetic products of different types of supernovae. With KAIT, we conduct the world's most successful search for nearby supernovae, having found over 300 in the past five years. We have spectroscopically classified hundreds of objects, providing a rich data base for individual and statistical studies. Our work shows that there are several physically distinct subclasses of hydrogen-poor (Type I) supernovae; some undergo thermonuclear runaway (Type Ia), while others explode via core collapse and rebound (Types Ib and Ic), as do hydrogen-rich (Type II) supernovae.
One of our group's major activities is to use supernovae as cosmological distance indicators. In 1998, we announced that high-redshift Type Ia supernovae seem to be dimmer (and hence farther away) than expected. This led us to conclude that the expansion of the Universe is accelerating, perhaps due to the cosmic "antigravity" effect of a nonzero vacuum energy density or some other type of "dark energy"! Deemed the top "Science Breakthrough of 1998" by Science magazine, we are now verifying this result through detailed studies of systematic effects such as cosmic evolution of supernovae and intergalactic extinction. We are also using supernovae to determine the equation of state (pressure versus density) of the repulsive dark energy. Our other studies of Type Ia supernovae are leading to improved measurements of Hubble's constant and bulk flows in the Universe. In addition, we are developing Type II supernovae as cosmological probes.
With KAIT, we are obtaining rapid follow-up optical observations of gamma-ray bursts (GRBs), in some cases earlier than anyone else in the world. This is leading to a better understanding of the physical nature of GRBs. We are also exploring the connection between GRBs and some types of core-collapse supernovae.
I am also interested in determining the physical properties of quasars and active galactic nuclei. The radiation emitted by a quasar is thought to be produced by matter being swallowed by a supermassive black hole, roughly a hundred million times more massive than the Sun, but the details are still murky. These photons ionize clouds of gas, and the resulting emission lines provide clues to the structure and nature of the central engine. I am especially fond of normal, nearby galaxies whose nuclei harbor activity similar to (but weaker than) that in classical active galaxies; at least some of these objects may have been luminous quasars in the distant past, but they are now accreting very little gas. I am part of a team that is finding supermassive black holes in the nuclei of nearby galaxies, some of which may be the remnants of long-dead quasars.
Another major research area is the search for black holes in X-ray binary star systems. In 1995, my students and I used Keck to find a convincing black hole candidate in our Galaxy; it has a minimum mass of 5 Suns. Since then we have found several others, and we are now searching for additional examples. The results will be used to study the complex evolution of binary star systems, the formation of black holes, and the structure of accretion disks around compact objects. The derived masses of the candidate black holes will be useful for strong-field tests of general relativity. Recently, I have become interested in the possible presence of intermediate-mass black holes (100 to 10,000 solar masses) in a minority of star clusters.