A Supergiant Supernova-Blown Bubble in the Spiral Galaxy NGC 1620
  We present UBR and H$\alpha$ imaging of NGC 1620, a highly inclined spiral galaxy that contains a large scale, arc-like feature of radius 3 kpc in its outer disk at a distance of $\sim$ 11 kpc from the center. What is unusual about this arc-like feature is its stellar nature and the presence of a luminous star cluster at its center. The arc is fragmented into HII region complexes and OB star clusters and shows two kinks in optical continuum light. It spans an angle of 220$^{\circ}$ on our U image and a full, though fragmented, circle on an unsharp masked R image. It is centered on a young star cluster that is the most luminous clump in blue optical continuum light besides the nucleus of the galaxy. This central star cluster has UBR colors and a surface brightness similar to those of other HII regions, but is a relatively weak H$\alpha$ emitter. It consists of at least three unresolved condensations in optical continuum light. Its location at the center of the arc and its prominence within the galaxy suggests that it has been the site of several generations of supernova explosions that swept up the surrounding gas into a supershell. When it attained a radius of $0.5-1$ kpc, this shell became gravitationally unstable and formed the stars which now delineate the arc. The constraints imposed by the survival of the expanding arc against random stellar motions and the age of the stars in the arc yield a required energy input by a minimum of 400 and a maximum of 6500 supernovae. In this scenario the asymmetry in surface brightness of the arc reflects the radial gradient of the gas density in the disk of NGC 1620, while the kinks reflect inhomogeneities in the original gas distribution with respect to the central star cluster. The supernova superbubble formed at least $5 \times 10^7$ yr ago so that, unless 
J. Patricia Vader and Brian Chaboyer