Much of my work over the past few years has been toward the study of barred disk galaxies. It has become evident with the availability of near-infrared imaging that a bar or oval distortion is a common feature of disk galaxies. The presence of such non-axisymmetries, whether triggered by an external interaction or formed through an internal global instability, will have significant evolutionary implications for a host galaxy. The torque produced by such features is an efficient mechanism to bring large quantities of disk gas into the inner regions, providing the required fuel for an Active Galactic Nucleus (AGN). On the other hand, bars create resonances which retard the inward flow of gas and promote vigorous star formation which may consume a large fraction of the gas. Bars may as well play a role in bulge building through the creation of vertical resonances. However, the build up of mass at the center weakens and will eventually destroy the bar. Though the interplay between these processes is still poorly understood, the overall effect is likely to be a bar driven morphological change from late to early disk type. It is just such observed changes which are at the center of the current debate on the evolution of galaxies over cosmological timescales.
Star formation and its associated feedback of energy through young stellar winds and supernovae also plays an important role in the evolution of galaxies. The spatial distribution and rate of star formation is greatly influenced by the local and global dynamics of the system, while the dynamics are continually modified by the star formation through the deposition of stellar mass and the flow of gas. This complicated coupling is poorly understood, yet is the key to future progress in understanding galaxy formation and evolution. The introduction of these fundamental processes into numerical simulations is in its infancy and represents the next major advancement in this field.
The ever increasing speed of workstations combined with modern algorithms has opened cutting edge research in galaxy evolution to even modest resources. Much of my plans for future numerical work will be accomplished using local resources. This has the additional advantage that students are able to easily participate.
The gas that collects in a circumnuclear ring may reach masses of a billion solar masses. In order to understand the consequences of having such a dynamically significant feature in a galactic disk, I have analytically derived the gravitational potential of a barred galaxy containing an elliptical ring of varying mass. Then using non-linear orbit analysis techniques I demonstrated that the gravity of the ring enhances the resonance, extending it to larger radii, and produces a feedback mechanism which promotes the formation of such massive rings. I also showed that significant observational signatures will be produced, such as truncation of any bar and twisting of isophotes.
Other recent work has included a project with one of my graduate students on the effects of gas inflow on the bending instability of disk galaxies. This instability is thought to give rise to the peanut shaped bulges found in many disk galaxies. We evolved fully self-consistent 3D models with my N-body/SPH code. We found that the presence of gas in the disk greatly damps the bending instability and weakens the bar through several different mechanisms, including the twisting of the family of orbits which supports the bar. This student is currently taking part in a collaboration between myself and Prof. E. Athanassoula from the Observatoire de Marseille to study the effects a pre-existing bar has on galaxy interactions.
My wife, who also holds an advanced degree in Astronomy, and I are studying the star formation properties of galaxies. For these studies I have developed a semi-automated algorithm to measure statistics of HII regions. The algorithm locates and defines HII regions by identifying critical contours which separate regions, taking into consideration the local diffuse background. We are collaborating on separate star formation studies with Prof. John Beckman (IAC) and Dr. Rebecca Koopmann (Yale). We are analyzing 65 H-alpha frames of disk galaxies, half of which are Virgo Cluster members and the rest a field control sample. Using this data set we plan to investigate the influence of environment on the massive star population and their spatial distributions.
My current work in collaboration with Prof. Shlosman deals with the dynamical decoupling of the self-gravitating inner stellar and gaseous regions of disk galaxies from their outer parts. Such decoupling is important in the understanding of the feeding mechanism that drives Active Galactic Nuclei (AGN). The conditions under which this occurs is dependent on the mass and viscosity of gaseous features, such as rings which form at resonances produced by m=2 asymmetries in the underlying stellar distribution. The viscosity is influenced by the rate of star formation and may help explain possible observed correlations between circumnuclear star formation and nuclear activity. This study focuses on the fundamental mechanisms that produce the structures which drive the evolution of the circumnuclear region of disk galaxies and represents the forefront of research in this area.
I am also currently developing formation models of galaxies. These models follow the evolution of a density enhancement and include the effects of star formation and feedback from winds and supernovae. The star formation algorithm takes into consideration the background field of stars and dark matter which modifies the stability criteria. The effect of intrinsic parameters such as initial virial temperature and angular momentum, and environmental parameters such as external pressure and background UV field are also being studied. Synthetic images in common wavelength bands along with color maps, metallicity gradients, and rotation curves are generated for comparison to observations. The inclusion of star formation and feedback sets these models apart from more contemporary works and represents the next stage of advancement with it's associated new discoveries in this fundamental area of astrophysics.