  The current picture of galaxy evolution advocates co-evolution of galaxies
and their nuclear massive black holes (MBHs), through accretion and merging.
Quasar pairs (6,000-300,000 light-years separation) exemplify the first stages
of this gravitational interaction. The final stages, through binary MBHs and
final collapse with gravitational wave emission, are consistent with the
sub-light-year separation MBHs inferred from optical spectra and
light-variability of two quasars. The double active nuclei of few nearby
galaxies with disrupted morphology and intense star formation (e.g., NGC 6240
and Mkn 463; ~2,400 and ~12,000 light-years separation respectively)
demonstrate the importance of major mergers of equal mass spirals in this
evolution, leading to an elliptical galaxy, as in the case of the double radio
nucleus (~15 light-years separation) elliptical 0402+379. Minor mergers of
galaxies with a smaller companion should be a more common occurrence, evolving
into spiral galaxies with active MBH pairs, but have hitherto not been seen.
Here we report the presence of two active MBHs, separated by ~430 light-years,
in the Seyfert galaxy NGC 3393. The regular spiral morphology and predominantly
old circum-nuclear stellar population of this galaxy, and the closeness of the
MBHs embedded in the bulge, suggest the result of minor merger evolution.
