Stellar Nuclear Rings in Barred Galaxies: Fossils of Past Circumnuclear Starbursts?

Abstract

We have found four barred S0 galaxies -- NGC 936, NGC 3945, NGC 4340, and NGC 4371 -- which contain smooth, luminous, purely stellar nuclear rings within their bars. These rings have little or no dust, no evidence for recent star formation, and are approximately the same color as surrounding bar and bulge. Thus, they are probably the aged remnants of bar-driven circumnuclear starburst episodes similar to those seen in barred galaxies today. Using kinematic data from long-slit spectroscopy, we construct rotation and resonance curves for two of the galaxies. In both cases, the nuclear rings appear to be located near or at the inner inner Lindblad resonances of the large-scale bars. We also discuss the difficulties inherent in detecting and identifying such rings, and show some of the surprising ways in which stellar rings can distort galaxy isophotes and ellipse fits.