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 Halpha 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. If the R-band light is dominated by old stars in the underlying density wave, then the results suggest evidence for a star-formation law which exhibits a threshold dependence on the strength of the density wave in the rings. Even if the R-band continuum in the ring is heavily contaminated with red light from the underlying young stars (a result at odds with the smooth continuum morphology of the ring) then a smaller, but still significant nonlinear enhancement in the star-formation rates in one segment of the second ring is required to explain the results. In either case, strong trends in the star-formation rate with azimuth around Ring 2 are in good agreement with off-center collisional ring galaxy models.