Star formation rates (SFR) in ring galaxies are typically 5 M yr-1, i.e., somewhat enhanced over large spirals but far below the SFRs inferred in LIRGs (cf. Appleton & Struck 1987; Higdon 1995; Higdon & Wallin 1997; Sanders & Mirabel 1996). However, the distribution of star formation is unique, being completely restricted to the expanding rings while simultaneously extinguished over the interior disk. Both effects are evident in the Lindsay-Shapley ring (L-S, hereafter) and Cartwheel, shown in Figure 1. A weak nuclear source is responsible for 5% of the star formation in both.
Figure 1. A polychromatic view of two ring galaxies: (top) the L-S ring galaxy (SFR = 8 M yr-1, Dring = 42 kpc) and (bottom) the Cartwheel (SFR = 21 M yr-1, Dring = 40 kpc). Massive star formation is restricted to the rings, which dominate emission at H, far-infrared, and radio continuum. The right-most column shows the neutral atomic ISM to be likewise confined to the rings, with low density gas filling the interior (Higdon et al. 2010).
The star forming rings are narrow, with slices showing very sharp radial cutoffs in H emission. This implies that OB stars remain in the rings for their Main Sequence lifetimes, which constrains the stellar velocity dispersion of the rings: * < rring / OB 45 km s-1. The weak line emission that is sometimes found in ring galaxy disks is post-starburst in origin, i.e., arising from aging HII complexes powered by A-stars.