Galaxies “are to astronomy what atoms are to physics” (from Sandage, A. 1961. The Hubble Atlas of Galaxies. Carnegie Institution of Washington). The research area of stellar dynamics has developed primarily as the study of galaxies as stellar systems, with significant extensions to smaller and to larger scales. Current progress is driven by recent or soon-to-be operational major initiatives, such as the astrometric mission Gaia, the James Webb Space Telescope, or the European Extremely Large Telescope.
Stellar dynamics addresses the mechanisms at the basis of the equilibrium, stability, and evolution of gravity-dominated systems. Collective phenomena present in self-gravitating systems originate spiral structure (as density waves) or warps (as bending waves) in galaxy disks or the structure of bright elliptical galaxies (as the result of collisionless incomplete violent relaxation). Much classical work on the morphology of stellar systems has been very successful, but many key issues still remain open. A revival of interest in some of these topics has been raised by the discovery of similar collective phenomena on the very small scale of star/planet formation and of protostellar disks.
As diagnostics, stellar dynamics has been instrumental in the discovery of dark matter halos in galaxies and still plays a key role in understanding the structure and dynamics of systems as large as clusters of galaxies. In terms of mechanisms, stellar dynamics touches on several fundamental issues in theoretical physics and in the statistical mechanics of many-body systems in mutual gravitational attraction, with the enormous complexity related to the formation of structures on all scales. It deals with phenomena and methods of investigations that are often in common with the study of plasma physics, in particular of astrophysical plasmas.
In addition to the study of very distant galaxies, hot topics in the field of the dynamics of galaxies comprise the structure and evolution of unusual systems, in particular low surface brightness galaxies, dwarf spheroidals, compact dwarfs, and ultra-diffuse galaxies and the structure of disks of satellites, such as those discovered around M31, NGC 5128, and the Milky Way Galaxy. These unusual systems are best studied in the nearby universe. They tend to pose challenging problems to the current cosmological paradigm.
Great progress is being made in our understanding of the internal structure and dynamics of globular clusters. Our Galaxy hosts more than 150 clusters, which turn out to be among the oldest stellar systems known. They are excellent laboratories for stellar dynamics. Their regularity and proximity make it possible to test in detail many important issues, such as the role of internal angular momentum, the tendency of weak collisional relaxation to generate mass segregation and partial equipartition, the non-trivial effects related to the tidal interactions with our Galaxy. Three hot topics are: the possible existence in globular clusters of Intermediate Mass Black Holes (as a small-scale analogue of the SMBHs hosted by galaxies; see the supermassive black hole in our Galaxy or in M87); the possible existence (or lack) in globular clusters of significant amounts of dark matter (in addition to the established presence of many dark remnants, resulting from the natural evolution of their stellar populations); and the origin of multiple components in their stellar populations reflecting the occurrence of different formation histories.
On these and other related topics our group has a long record of research activity.