Multiwavelength observations of collisional ring galaxies. III. Oxygen/nitrogen abundances and star formation properties of ring knots

This is the third paper in a series examining the multiwavelength properties of a sample of northern ring galaxies, in which we present optical long-slit spectra and broadband BVR and JHK colors of individual star-forming knots embedded in the ring. We present the oxygen and nitrogen abundances of the starburst knots and compare the reddening-corrected colors of individual star clusters with recent models of stellar evolution. We also present kinematic data for one system, LT 41. The star-forming knots in the rings have very similar colors, which implies similar ages in the range 4-80 Myr. These ages are less than the dynamical ages of the rings, which are typically 100-300 Myr. The fact that the implied ages of knots within individual ring galaxies cluster around the same age argues that their formation is recently triggered by a coherent event all around the ring. This strongly supports the triggering of the star formation as being due to the propagation of a radial density wave, as in the classical picture. The ring galaxies are found to have subsolar metallicities in the range one-half to one-fifth solar in [O/H] and [N/H] ratios. There is a suggestion of an increase in the mean nitrogen abundance for the rings of larger linear size, but oxygen abundances show no trend with ring diameter. The uniformity of the oxygen abundances over the sample is consistent with their rather narrow range of K-band luminosities, which suggests that even in strongly disturbed galaxies, when prompt local enrichment is expected to be important, the primary factor governing oxygen metallicity is total luminosity.