The unusual emission line spectrum of IZw1

Most Seyfert I galaxies show strong Fe II lines in their spectrum having the velocity and width of the broad emission lines. To remove the Fell contribution in these objects, an accurate template is necessary. We used very high signal-to-noise, medium resolution archive optical spectra of I Zw I to build such a template. I Zw I is a bright narrow-line Seyfert 1 galaxy. As such it is well suited for a detailed analysis of its emission line spectrum. Furthermore it is known to have a very peculiar spectrum with, in addition to the usual broad and narrow line regions, two emission regions emitting broad and blue shifted [O III] lines making it a peculiarly interesting object. While analysing the spectra, we found that the narrow-line region is, unlike the NLR of most Seyfert I galaxies, a very low excitation region dominated by both permitted and forbidden Fell lines. It is very similar to the emission spectrum of a blob in eta Carinae which is a low temperature (T-e similar to 6500 K), relatively high density (N-e = 10(6) cm(-3)) cloud. The Fe II lines in this cloud are mainly due to pumping via the stellar continuum radiation field (Verner et at. 2002). We did not succeed in modelling the spectrum of the broad-line region, and we suggest that a non radiative heating mechanism increases the temperature in the excited H I region, thus providing the necessary additional excitation of the Fell lines. For the low-excitation narrow-line region, we are able to apply boundaries to the physical conditions accounting for the forbidden and permitted Fe II lines (10(6) < N-e < 10(7) cm(-3); 10(-6) < U < 10(-5)).