Threshold Star-Formation Effects in the Peculiar Galaxy ARP 10 (= VV 362)

We present images of the peculiar galaxy Arp 10 which reveal two rings of concurrent star formation. Apart from a bright ring visible on early photographs of the system, an even brighter inner ring of H II regions is found within the nuclear bulge of the galaxy. A very faint ring-arc of H II regions is also seen associated with a third outer ring or shell. We detect a small companion galaxy on the minor axis of Arp 10 and an extended extra-nuclear knot which lies between the first and second rings. An investigation of the Hα fluxes in the rings reveals an increase in the emission where the ring surface density in the R-band light exceeds 21.3±0.2 mag arcsec -2