Most Seyfert 1s show strong Fe II lines in their spectrum having the velocity and width of the broad emission lines .
To remove the Fe II contribution in these objects , an accurate template is necessary .
We used very high signal-to-noise , medium resolution archive optical spectra of I Zw 1 to build such a template .
I Zw 1 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 1 galaxies , a very low excitation region dominated by both permitted and forbidden Fe II lines .
It is very similar to the emission spectrum of a blob in \eta Carinae which is a low temperature ( T _ { e } \sim 6 500 K ) , relatively high density ( N _ { \it 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 al . [ ] ) .
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 Fe II lines .
For the low-excitation narrow-line region , we are able to settle 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 } ) .