Evidences of merging in the Seyfert galaxy NGC 3393 revealed by   modelling the spectra
  The discovery of two active black holes in the Seyfert galaxy NGC 3393, separated by about 490 light years, revealed a merging event. This led us to look for other evidences of galaxy collision and merging through the analysis of the observed spectra in different frequency ranges. We found preshock densities higher by a factor of about 10 in the NGC 3393 NLR than in other AGN and patches of ionized matter beyond the observed NLR bulk. They can be explained by compression and heating of the gas downstream of shock waves created by collision. Metallicity in terms of the O/H relative abundance, is about 0.78 solar. Mg/H depletion by a factor of about 3 compared with solar cannot be explained by Mg trapping into dust grains, due to rather high shock velocities. The low O/H and Mg/H abundances indicate mixing with external matter during collision. Twice solar N/H is predicted by modelling the spectra of high shock velocity clouds reached by a Ts =8.6 10^4 K black-body flux. This suggests that Wolf-Rayet stars could be created by galaxy collision in the central region. 
M. Contini
astro-ph.GA