THE MERGING SEYFERT-GALAXIES MKN-423 AND MKN-739

In this paper long slit spectroscopy and broad band imaging of the int ermediate Seyfert galaxies Mkn 423 and Mkn 739 are presented. The anal ysis of the extranuclear emissions reveals that Mkn 423 and Mkn 739 ar e undergoing a strong burst of star formation spread over a kpc-sized scale. Although the morphological appearance of Mkn 423 is suggestive of a galaxy of type Sa/SBa with a giant HII region (component B), the peculiar kinematics of Mkn 423 and the very large Halpha luminosity of component B suggest that this system could be the result of an ongoin g merging of two galaxies. The resolution almost-equal-to 2 angstrom F WHM of the spectra taken in the range 4000-5500 angstrom allowed to re solve substructures in the Hbeta and [OIII]lambdalambda4959,5007 line profiles, permitting an analysis of the circumnuclear regions of these objects. The narrow line spectrum of Mkn 423 originates in two partia lly resolved blobs, of high ionization degree, which are likely to be the optical counterpart of two radio emitting sources or which are ass ociated with the NLR and circumnuclear gas ionized by the central sour ce. Mkn 739 shows extended high-ionization gas up to a distance almost -equal-to 4 h-1 kpc from the Seyfert nucleus. This gas is photoionized by the active nucleus itself, and it is in a kinematical status (prob ably in outflow) different from that of the low ionization gas associa ted with star formation. The high ionization degree observed as far as congruent-to 2 h-1 kpc from the Seyfert nucleus suggests that the cen tral ionizing source is most probably anisotropic.