The following are the projects that I have been worked on as the principal investigator.

• Quantifying the impact of galaxy mergers on the stellar distribution in galaxies

Galaxy-galaxy interaction is one of the primary drivers of galaxy evolution over cosmic time. To assess the effects of galaxy mergers on the size and morphology of galaxies, I performed N-body simulations to study galaxy mergers between two disk galaxies with different relative orientations and relative velocities (Wu & Jiang 2009).

• Modeling the formation and evolution of collisional ring galaxies

Ring galaxies are galaxies that present an unusual morphology in that rings are observed as dominating parts of their structures. Collisional ring galaxies provide excellent laboratories for studying the effects of galaxy collisions on the galaxy formation and evolution due to their simple interaction geometry.

Although many studies have been conducted to show ring galaxies can form during the collisions between an intruding galaxy and a disk galaxy, the association between different parameters of the collisions and the shape of ring galaxies was not clear. To establish this link, I carried out N-body simulations and investigated the formation and evolution of ring galaxies through axisymmetric and non-axisymmetric collisions between a dwarf galaxy and a disk galaxy (Wu & Jiang 2012, 2015). Fig. 1 shows an example at a time after two galaxies collide with each other. Because this is a non-axisymmetric collision, an elliptical ring forms and the center of the disk galaxy shifts.

• Studying the evolution and dynamics of double-barred galaxies

Bars are one of the most common non-axisymmetric structures in disk galaxies and affect the evolution of galaxies dramatically by redistributing mass and angular momentum. The major focus of my research is to understand the formation, evolution and effects of bars in galaxies. Specifically, I investigate the galaxies which have two bars, that is an inner secondary bar embedded within an outer primary bar. They are so-called double-barred galaxies. Fig. 2 is an example showing the morphology of the double-barred galaxy NGC 3504. The investigation of double-barred galaxies is especially important as the inner bar is expected to drive gas inflow towards the nucleus of galaxies to feed supermassive black holes and therefore significantly change the evolution of galaxies (Shlosman et al. 1989).

With high-resolution N-body simulations, I and my collaborators explored the parameter space and successfully generated the long-lived double barred galaxies, as shown in Fig. 3. Furthermore, we developed three methods to quantify the characteristics of the inner and outer bars, including the size, strength, and instantaneous mode/pattern speed, and thus can study the time evolution of the bars (Wu, Pfenniger & Taam 2016, 2018). Our results show the presence of chaos in galaxies due to the strong time-dependent evolution of the two bars. It affects the motion of stars and is expected to reconstruct the chemical distribution in galaxies. Ultimately, our work serves as a basis for evaluating the characteristics and lifetime of bars in galaxies.