The Ring Produced by an Extra-Galactic Superbubble in Flat Cosmology
  A superbubble which advances in a symmetric Navarro--Frenk--White density
profile or in an auto-gravitating density profile generates a thick shell with
a radius that can reach 10 kpc. The application of the symmetric and asymmetric
image theory to this thick 3D shell produces a ring in the 2D map of intensity
and a characteristic `U' shape in the case of 1D cut of the intensity. A
comparison of such a ring originating from a superbubble is made with the
Einstein's ring. A Taylor approximation of order 10 for the angular diameter
distance is derived in order to deal with high values of the redshift.
