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Keywords:

  • radiation mechanisms: general;
  • shock waves;
  • ISM: abundances;
  • galaxies: individual: NGC 3393;
  • galaxies: Seyfert

ABSTRACT

The discovery (Fabbiano et al. 2001) of two active black holes in the Seyfert galaxy NGC 3393, which are separated by about 490 light years, has revealed a merging event. This has led us to look for other evidence of galaxy collision and merging, using an analysis of the observed spectra in different frequency ranges. In the narrow-line region (NLR) of NGC 3393, we have found pre-shock densities that are higher by a factor of about 10 than in other active galactic nuclei, and we have found patches of ionized matter beyond the observed NLR bulk. These can be explained by the compression and heating of the gas downstream of the shock waves created by the collision. Metallicity, in terms of the O/H relative abundance, is about 0.78 solar. The Mg/H depletion by a factor of about 3, compared with solar, cannot be explained by Mg trapping into dust grains, as a result of high shock velocities. The low O/H and Mg/H abundances indicate mixing with external matter during the collision. Twice solar N/H is predicted by modelling the spectra of high-shock-velocity clouds reached by a T* ≤ 105 K blackbody flux. This suggests that Wolf–Rayet stars could be created by galaxy collision in the central region.