The ultraviolet properties of the narrow-line quasar I Zw 1

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 Angstrom emission of I Zw 1 using a high signal-to-noise (50-120) spectrum obtained with the Faint Object Spectrograph of the Hubble Space Telescope. 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 that 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 similar to 1870 km s(-1) and velocity dispersion of less than or similar to 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 similar to 2000 km s(-1) for He II lambda 1640. This may indicate an outflowing 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 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) similar to 10(11) cm(-3), i.e., about an order of magnitude larger than that implied by C III] lambda 1909 in most quasars. A BLR component with a density n(e) greater than or similar to 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 Angstrom and 1871 Angstrom. These three features have been proposed by Johansson & Jordan as evidence for significant Ly alpha pumping of the 8-10 eV levels of Fe II. 7. Significant Fe II 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 need to be explored. 8. Very weak [C III] lambda 1907 and [Si III] lambda 1883 emission may be present, suggesting a narrow-line-region component with n(e) similar to 5 x 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 and as a useful benchmark for photoionization models, in particular, for models of the complex Fe II emission spectrum.