Galactic Rings: Signposts of Secular Evolution in Disk Galaxies

This conference will be held at The University of Alabama, Bryant Conference Center in Tuscaloosa, AL May 27-June 1, 2018.

Key Topics

  • Bars, spirals and rings in disk galaxies
  • Secular evolution and dynamics of ringed galaxies
  • AGN, star formation and feedback connections
  • Polar, collisional, and accretion rings
  • Rings in a cosmological context
  • Rings in Local Group galaxies
  • Shocks, Lindblad Resonances and manifolds
  • Formation and destruction of rings and bars


Scientific Rationale

Galaxy morphology presents many puzzles as we seek to understand how galaxies form and evolve. The role of processes such as internal perturbations, external gas accretion, major and minor interactions, star formation and feedback processes, environmental density, and the dark matter halo on galactic structure are all issues that have relevance to different aspects of galactic evolution. If we consider that galaxy morphology can and likely does change over timescales much less than a Hubble time, then there is reason to examine specific structures to gauge their significance to the evolutionary process.

A type of feature that has not previously been the focus of a major meeting is galactic rings. Many normal disk-shaped galaxies show characteristic ring-like patterns in their luminosity distributions. These disk rings are most abundant in early-to-intermediate type spiral galaxies, and are often part of the spiral structure. In the local Universe, ~20% of disk galaxies include a ring and an additional 1/3rd have broken or partial rings. Disk rings are further divided into three categories - nuclear (or circumnuclear), inner, and outer – that are recognized mainly from their relation to bars and ovals. All three ring types have distinctive intrinsic shapes and orientations relative to bars, and all show a wide range of star-forming properties that in some galaxies appears directly tied to intrinsic ring shape. It is also possible for all three ring types to co-exist in the same galaxy. The HI gas distribution in galaxies and profile breaks have been found to be influenced by disk rings, and several studies have noted a preponderance of ring, pseudoring, and bar features in Seyfert galaxies. Star formation is an important aspect of disk rings that has not received the attention it deserves. In some normal galaxies, a ring is the only place where star formation is occurring, while in others, a bright ring may show little or no recent star formation. Some nuclear rings are best described in terms of a starburst. The galaxy ring population also includes environmentally-driven structures like collisional ring galaxies and polar ring galaxies that are believed to have been formed in disruptive collisions. These often unusual-looking rings are much rarer than disk rings but can sometimes resemble disk rings in certain observed orientations. 

The presence of internal perturbations and even external influences can produce slow, secular changes in the morphology of galaxies over time periods of a few Gyr. This is the basis for the secular evolution paradigm, which is the prime theoretical framework we use to examine the structure and nature of galactic disk rings and their host galaxies. Although dynamical orbit resonances have been the principal interpretation of galactic disk rings, this idea has not gone unchallenged. Many galaxies do not show rings, yet likely still have resonances in their disks. In recent years, alternative views to the resonance idea have been proposed, the most significant being the invariant manifold theory, where rings are linked to unstable orbits near the L1 and L2 Lagrangian points in the bar potential. The two views – resonances and manifolds – are so different, and at the same time so comparably successful in many respects, that they merit discussion at a scientific meeting.

The main goal of this meeting is to discuss, from a variety of approaches, what galactic rings have to offer for our understanding of galaxy evolution. We ask questions like these: 

  • Why has there been greater success is understanding the structure and dynamics of ringed galaxies than of non-ringed galaxies?
  • How can resonant interpretations of rings be examined beyond the issue of morphology? Is the resonance idea still viable?
  • Can evidence of bar pattern speed evolution be detected in the properties of galactic disk rings?
  • How do observed ring properties, such as detailed morphologies, intrinsic shapes, orientations of major axes relative to bars, and relative sizes connect to dynamics?
  • How do rings form and evolve? Can ring evolution explain the existence of other structures, like lenses? How might ring evolution be tied to bar evolution?
  • Are bars the essential element in ring formation we have thought them to be? Why are rings and lenses found in nonbarred galaxies?
  • What light can rings shed on the nature of spiral structure itself?
  • What role does environment play on the existence and morphology of galactic rings?
  • Where does the gas in the highly-organized star forming rings of some early-type galaxies come from?
  • What kinds of stars and stellar populations are found in galactic rings, and what determines the rate of star formation?
  • How strongly does disk environment (nuclear/inner/outer rings, bar, spiral arms etc) affect SFR and star/cluster formation efficiency?
  • Why are so many galaxies multi-ringed? How can rings having very different timescales coexist in the same galaxy? What distinguishes single-ring galaxies from multi-ring galaxies?
  • How can ringed galaxies be used as "stepping stones" to further our understanding of other types of galaxies?
  • Do rings represent a singular path in disk galaxy evolution, or can multiple paths lead to such galaxies?
  • What cosmological perspectives can we gain from the study of galactic rings?
  • How well can IFU observations and simulations resolve the role of rings and other secular structures?


Other issues concern the existence of purely gaseous rings, dust rings, blue and red ansae, UV-bright rings, large-scale magnetic fields in rings,  galactic rings in the local neighborhood and at high redshift, metallicity gradients, the presence of AGN in ringed galaxies, and ringed galaxies in cosmological galaxy simulations. Although the main emphasis of the meeting will be on galactic disk rings, encounter-driven rings are an important topic because these represent features that can develop in rare types of collisions. Polar rings especially can resemble disk ring galaxies if viewed in certain orientations. Related to encounter-driven rings are accretion rings, possibly formed by externally-accreted gas. 

This meeting will bring together experts of galactic and environmentally-driven rings as well as other secular structures to further our understanding of galaxy evolution.