Letters of Intent received in 2016

LoI 2018-1966
Galactic Rings

Topics

1. systematic morphology and relation to other galactic structures
2. star formation timescales and stellar populations in rings
3. purely gaseous rings, dust rings, and nonbarred ringed galaxies
4. circumnuclear starburst rings, shocks and AGN
5. kinematics and dynamics of ringed galaxies
6. numerical simulations of ring formation and evolution
7. resonance rings, manifold rings, and cataclysmic rings
8. environmental factors and influences
9. high redshift rings
10. disk evolutionary implications and cosmological significance

Rationale

Galaxy morphology presents many puzzles as we seek to understand how
galaxies form and evolve. The role of many effects on galactic
structure, such as internal perturbations, gas accretion, major and
minor interactions, star formation and feedback processes,
environmental density, and the dark matter halo, are all issues that
have relevance to different aspects of galactic evolution. If we
consider that galaxy morphology must evolve with time, then there is
reason to examine specific morphological structures in detail 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 a wide range of star-forming properties that in some
galaxies appears directly tied to intrinsic ring shape. The HI gas
distribution in galaxies and profile breaks have been found to be
influenced by disk rings. Several studies have also noted a
preponderance of ring, pseudoring, and bar features in Seyfert
galaxies.

The frequent presence of bars and ovals in galaxies with disk rings is
strong enough to suggest a cause and effect relation that ties galactic
rings to processes of secular evolution. The most popular
interpretation, dating at least to the early 1960s, is that rings are
visible manifestations of galactic resonances, the main resonances
being the inner Lindblad resonance for nuclear rings, the inner 4:1
resonance for inner rings, and the outer 4:1 and outer Lindblad
resonances for outer rings. These are regions where orbits cross in the
potential of a bar, and where gravity torques collect gas into roughly
orbit-shaped features. As noted by Buta and Combes in a 1996 review article,
"identifying the locations of the main resonance regions in barred galaxies is
one of the most important steps in understanding the dynamics of these objects."
Disk rings have also been interpreted in terms of invariant
manifolds of orbits emanating from unstable Lagrangian points in the
gravitational potential of a bar. The two interpretations are very
different but can almost equally well account for the morphology and
structure of rings.

In spite of considerable advances in instrumentation and in the
sophistication of numerical simulations and dynamical modeling, there
are many aspects of galactic rings that are poorly understood. An
example is why, given the widespread presence of internal
perturbations, only some disk galaxies have rings, while many do not.
How can resonant interpretations of rings be examined beyond the issue
of morphology? Is the resonance idea still viable? What role does
environment play on the structure of rings? Do rings represent a
singular path in disk galaxy evolution, or can multiple paths lead to
such galaxies? What kinds of stars are found in galactic rings, and
what determines the rate of star formation? How do rings achieve their
intrinsic shapes and orientations? Other issues concern the existence
of purely gaseous rings, dust rings, blue and red ansae, UV-bright rings in
nonbarred galaxies, large-scale magnetic fields in rings, metallicities within
and adjacent to rings, galactic rings in the local neighborhood, at
high redshift, and in rich galaxy clusters, starburst nuclear rings,
and the presence of AGN in ringed galaxies.

The interpretation of galactic rings is complicated further by the fact
that similar features to disk rings can form in violent collisions
between galaxies or, possibly, via accretion of cold gas from filaments
of the intergalactic medium. Accretion ring galaxies, polar ring
galaxies, and inclined ring galaxies (whose rings are collectively
referred to as ``cataclysmic" rings) represent very different phenomena
from disk rings and in some cases can masquerade as disk rings
depending on viewing geometry. Polar rings provide an interesting
avenue to probe dark matter halo shapes. Thus, correct recognition is
essential to interpreting ring phenomena in galaxies.

An expanded version of the rationale will be included in the final proposal, as well as a complete SOC. The IAU GA Venue will allow a wide international participation in the conference.