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The prominent broad emission lines of quasars are typically blended with weaker emission features (e.g. Fe~II multiplets). This blending prevents a reliable study of the prominent line profiles, the measurement of weaker lines, and the identification of very weak lines. Blending effects are minimized in the rare class of narrow line quasars (i.e. with FWHM$\sim 1000$~km~s$^{-1}$). Such objects therefore provide us with the opportunity to make a significantly more accurate study of quasar emission line properties.
We report here the results of an HST study of I Zw 1 which is both a narrow line quasar and a very strong Fe~II emitter. High S/N spectra (50-120 per resolution element) were recently obtained with the FOS from 1150\AA\ to 3250\AA. The spectra are very rich in emission lines. There is essentially no wavelength range free of emission features. Preliminary analysis indicates the presence of significant C~III$^*~\lambda 1176$ emission, and possibly also Fe~III (UV 34), together with a large number of strong Fe~II UV multiplets, and other emission features whose likely identification is discussed.
The Mg~II~$\lambda\lambda 2796, 2804$ and the Si~IV]~$\lambda\lambda 1394, 1403$ doublets are resolved, which allows constraints on the optical depth in these lines. We also compare the prominent line profiles and find significantly different line peak velocities for some of the lines.
We plan to use the I Zw 1 Fe~II spectrum as a template for measuring the UV Fe~II emission in HST spectra of other quasars with broader lines. This will allow us to study in detail the UV Fe~II emission properties of quasars and their relation to the optical Fe~II emission. The subtraction of the Fe~II emission will allow a more accurate measurement of weak lines, and of strongly blended lines, such as Mg~II, C ~III]~$\lambda 1909$ and He~II~$\lambda 1640$.
Support for this work is provided by NASA through Grant No. GO-5486.01-93A from the Space Telescope Science Institute.
