  We present a novel algorithm to detect double nuclei galaxies (DNG) called
GOTHIC (Graph BOosted iterated HIll Climbing) - that detects whether a given
image of a galaxy has two or more closely separated nuclei. Our aim is to
detect samples of dual or multiple active galactic nuclei (AGN) in galaxies.
Although galaxy mergers are common, the detection of dual AGN is rare. Their
detection is very important as they help us understand the formation of
supermassive black hole (SMBH) binaries, SMBH growth and AGN feedback effects
in multiple nuclei systems. There is thus a need for an algorithm to do a
systematic survey of existing imaging data for the discovery of DNGs and dual
AGN. We have tested GOTHIC on a known sample of DNGs and subsequently applied
it to a sample of a million SDSS DR16 galaxies lying in the redshift range of 0
to 0.75 approximately, and have available spectroscopic data. We have detected
159 dual AGN in this sample, of which 2 are triple AGN systems. Our results
show that dual AGN are not common, and triple AGN even rarer. The color (u-r)
magnitude plots of the DNGs indicate that star formation is quenched as the
nuclei come closer and as the AGN fraction increases. The quenching is
especially prominent for dual/triple AGN galaxies that lie in the extreme end
of the red sequence.
