  IZw1 is the prototype narrow line quasar. This type of AGNs show a number of
common peculiar characteristics whose physical origin is not yet well
understood. Here, I review the results of a detailed study by Laor et al.
(1997b) of the UV emission properties of IZw1, based on a high quality HST FOS
spectrum. The main results are: 1) The Mg II and the Al III doublets are
resolved, indicating optically thick thermalized emission, as expected for a
standard BLR. 2) Lines from ions of increasing ionization level show increasing
excess blue wing flux, and an increasing line peak velocity shift, suggesting
an out-flowing component of the BLR, visible only in the approaching direction,
where the ionization level increases with velocity. 3) A weak associated UV
absorption system is detected in N V, C IV, Si IV and Lya, which is probably
part of the outflow suggested above. 4) The C III] 1909 line indicates a BLR
density about an order of magnitude larger than commonly observed in quasars.
An additional denser component is implied by the Al III 1857 doublet. 5)
Prominent Fe II and FeIII emission is seen, including components which may
indicate significant Lya pumping of the 8-10 eV levels of Fe II. Further
studies suggest that some of the above properties are typical of narrow line
AGNs.
