Inner rings in disc galaxies: dead or alive

Abstract

In this Letter, I distinguish "passive" inner rings as those with no current star formation as distinct from "active" inner rings that have undergone recent star formation. I built a sample of nearby galaxies with inner rings observed in the near-and mid-infrared from the NIRS0S and the S(4)G surveys. I used archival far-ultraviolet (FUV) and H alpha imaging of 319 galaxies to diagnose whether their inner rings are passive or active. I found that passive rings are found only in early-type disc galaxies (-3 <= T <= 2). In this range of stages, 21 +/- 3% and 28 +/- 5% of the rings are passive according to the FUV and H alpha indicators, respectively. A ring that is passive according to the FUV is always passive according to H alpha, but the reverse is not always true. Ring-lenses form 30-40% of passive rings, which is four times more than the fraction of ring-lenses found in active rings in the stage range -3 <= T <= 2. This is consistent with both a resonance and a manifold origin for the rings because both models predict purely stellar rings to be wider than their star-forming counterparts. In the case of resonance rings, the widening may be at least partly due to the dissolution of rings. If most inner rings have a resonance origin, I estimate 200 Myr to be a lower bound for their dissolution time-scale. This time-scale is of the order of one orbital period at the radius of inner rings.