Jerry A. Sellwood

Department of Physics & Astronomy
Rutgers, The State University of New Jersey
136 Frelinghuysen Road
Piscataway, NJ 08854-8019

Telephone: (732) 445 5287
Fax: (732) 445 4343
E-mail: sellwood_at_physics.rutgers.edu

Some biographic information on Sellwood is available here.

Teaching

Research Interests

My main interests are structure and evolution of galaxies, their formation and their dark matter content. The following are some of the topics I have been working on recently:

• Dark Matter The extended flat rotation curves of spiral galaxies provide some of the strongest evidence for mass discrepancies in galaxies, and are usually interpreted as evidence for dark matter halos. My research suggests that dark matter halos have large low density cores, i.e. most of the dark matter lies in the outskirts of galaxies and that the inner parts of galaxies are dominated by the mass in the stars. This point of view conflicts with Cold Dark Matter cosmology which predicts high central densities for the halos. One idea to resolve this conflict has been to suppose that the Dark Matter particle has a low-enough mass that thermal velocities were significant at the time galaxies form (the Warm Dark Matter model). I show that this idea seems to be ruled out, and that the Dark Matter particle, if one exists, must have unusual properties.
• Spiral Structure Most disc galaxies display graceful spiral patterns, yet over a century and a half, after their first discovery, we still do not fully understand how they originate! My view is that the spirals are short-lived, recurring structures caused by real instabilities. Some support for this point of view has recently been obtained by the Hipparcos satellite, launched by ESA.
• Bar Stability About 30% of galaxies are strongly barred and a roughly equal fraction are weakly barred, while the remaining significant minority do not have bars. We do not yet understand the reasons for these proportions. It has been suggested that galaxies with less dark matter develop bars through a global instability, but there is no evidence to support this speculation. My work suggests that whether a galaxy is barred or not today may depend on the distribution of angular momentum in the material from which it formed.
• Formation of massive Black Holes and QSO activity I argue that a bar must develop early in the life of nearly every galaxy and that gas to create and fuel the QSO is driven into the center of the galaxy by the bar. The QSO lifetime, and the mass of its central engine, are also controlled by large-scale dynamics, since the fuel supply is shut off after a short period by the development of an inner Lindblad resonance. This resonance causes the gas inflow along the bar to stall at a distance of a few hundred parsecs from the center. The ILR develops as a result of previous inflow, making quasar activity self-limiting. The bars are weakened and can be destroyed by the central mass concentration formed in this way.
• Warps Many disk galaxies are not perfectly flat, but the outer parts are bent away from the plane defined by the inner disk, especially in the gas. Theoretical work suggests that this state of affairs could not persist for the believed ages of galaxies and that some mechanism is required to excite the warp continually (or repeatedly). With my student, Juntai Shen, we show that realistic warps can be excited by late infalling matter having a misaligned angular momentum vector. Our simulations manifested realistic warps, and a series of simplified experiments allowed us to understand the complex dynamical interaction of the disk and halo with the externally applied torque. Furthermore, we were able to show that the persistence of warps is not as puzzling as is widely believed; halo damping of the warp is very weak since the inner halo remains aligned with the slowly precessing disk and is only weakly coupled to the outer halo. While these idealized experiments have not settled all outstanding questions, they have shown that the problem may be closer to a solution than is widely thought.
• Numerical Methods for N-body simulations I have compared five different methods for simulating galaxy models, which shows that grid based (or particle-mesh) methods are superior in efficiency to any other and are quite versatile. The combination of efficient codes and quiet starts allows many galactic dynamical problems to be tackled without the need for supercomputers.