1.1. Why Study Rings?
Ring galaxies are rare amongst samples of bright galaxies. Despite
this, we believe that ring galaxies are important for the following
- Ring galaxies represent a class of colliding galaxies in which the
disk of at least one of the participants of the collision remains
sufficiently intact after the collision to allow its structure and
star formation properties to be studied in detail. This is not often
the case in colliding galaxy systems. In a sense, the ring galaxy
serves as a galaxy-scale perturbation experiment that allows us to
study the hydrodynamic and stellar evolutionary processes within the
host disk. In many cases, the gravitational perturbation of the small
companion galaxy is expected to be modest, driving low amplitude waves
through its disk. In other cases the companion is more substantial,
and large three-dimensional effects and strong non-linear behavior is
predicted. Models of rings have now been developed by a number of
independent groups, and such tools can be used to make predictions
which can be compared with observation.
- Ring galaxies are the sites of vigorous non-nuclear star formation.
Typical knots in ring galaxies have optical luminosities similar to,
or in excess of, the Large Magellanic Cloud. The colors and spectra
and extreme luminosity of the HII regions in the rings suggest large
numbers of OB stars are being born at high rates. Ring galaxies also
have generally high global far-infrared (IR) luminosities and large
far-IR color temperatures compared with "normal" galaxies. Recent
observations also show that they contain substantial quantities of
molecular gas. These properties are shared with nuclear starburst
- Ring galaxies can provide a means of studying the time evolution of
star formation across and inside the ring. The simplest models predict
that stars form in the expanding ring. A star formation burst formed
at the leading edge of the expanding density wave is expected to
produce a monotonic color gradient in the wake of the ring, as
evolving stars drift inwards from their birth sites. There is now
mounting evidence for such color gradients and other indicators of
stellar evolution behind the ring.
- Models indicate that off-center, moderate impact-parameter
collisions often produce a strongly asymmetric ring initially, and
many known ring galaxies appear to fall into this
category. Interesting azimuthal and vertical variations are predicted
from the models in these cases and these can provide interesting tests
of the mechanisms of star formation which can be compared with
- Because of its geometrical simplicity, as compared with more
dynamically messy galaxy mergers, the collisional ring galaxy can be
used to explore differences between the behavior of gas and stars in
collisions. Dissipation can aid the development of local gravitational
instabilities in the gas, causing the collapse and shearing of gas
complexes, star formation and perhaps the formation of `spokes'
similar to those found in the Cartwheel.