I Zwicky 1 ( I Zw 1 ) is the prototype narrow line quasar .
Its narrow line profiles minimize blending effects and thus allow a significantly more accurate study of individual quasar emission lines .
We report here the results of our study of the 1150-3250 Å emission of I Zw 1 using a high S/N ( 50-120 ) spectrum obtained with the Faint Object Spectrograph ( FOS ) of the Hubble Space Telescope ( HST ) .
The UV spectrum of I Zw 1 is very rich in emission lines , with essentially no wavelength range free of emission line features .
The following main new results are obtained :

1 . The Mg II \lambda 2798 doublet is partially resolved , and the measured doublet ratio is 1.2/1 .
This ratio verifies theoretical predictions that the Mg II \lambda 2798 line is thermalized in the Broad Line Region ( BLR ) .

2 . The Al III \lambda 1857 doublet is clearly resolved , with a thermalized doublet ratio of 1.25/1 .
The line optical depth provides an upper limit to the clouds distance from the ionizing source which is consistent with the “ standard ” BLR radius .

3 . A weak associated UV absorption system is detected in N V ( possibly also in C IV and Ly \alpha as well ) , suggesting an outflow with a velocity of \sim 1870 km s ^ { -1 } and velocity dispersion \lesssim 300 km s ^ { -1 } .

4 . Lines from ions of increasing ionization level show increasing excess blue wing flux , and an increasing line peak velocity shift , reaching a maximum blueshift of \sim 2000 km s ^ { -1 } for He II \lambda 1640 .
This may indicate an out-flowing component in the BLR , where the ionization level increases with velocity , and which is visible only in the approaching direction .
The highest velocity part of this outflow may produce the associated UV absorption system .

5 . The small C III ] ~ { } \lambda 1909 equivalent width ( EW ) , and the small C III ] ~ { } \lambda 1909 / Ly \alpha and C III ] ~ { } \lambda 1909 / Si III ] ~ { } \lambda 1892 flux ratios indicate a typical BLR density of n _ { e } \sim 10 ^ { 11 } cm ^ { -3 } , i.e .
about an order of magnitude larger than implied by C III ] ~ { } \lambda 1909 in most quasars .
A BLR component with a density n _ { e } \gtrsim 10 ^ { 11 } cm ^ { -3 } is implied by the EW and doublet ratio of the Al III \lambda 1857 doublet .

6 . Prominent Fe II UV 191 emission is seen , together with weaker line emission at 1294 Å and 1871 Å .
These three features have been proposed by Johansson & Jordan ( 1984 ) as evidence for significant Ly \alpha pumping of the 8-10 eV levels of Fe II .

7 . Significant Fe III emission is present .
The Fe III UV 34 and UV 48 multiplets are clearly resolved , and Fe III UV 1 , UV 47 , UV 50 , and UV 68 may also be present .
The implications of significant Fe III emission for the conditions in the BLR needs to be explored .

8 . Very weak [ C III ] ~ { } \lambda 1907 and [ Si III ] ~ { } \lambda 1883 emission may be present , suggesting a Narrow Line Region ( NLR ) component with n _ { e } \sim 5 \times 10 ^ { 5 } cm ^ { -3 } .

The rich UV emission spectrum of I Zw 1 should serve as a useful “ template ” for the identification of weak features in other Active Galactic Nuclei ( AGNs ) , and as a useful benchmark for photoionization models , in particular for models of the complex Fe II emission spectrum .