Space Telescope Imaging Spectrograph echelle observations of NGC 4151: Variable ionization of the intrinsic UV absorbers

We present echelle observations of the intrinsic UV absorption lines in the Seyfert galaxy NGC 4151; these observations were obtained with the Space T elescope Imaging Spectrograph aboard the Hubble Space Telescope on 1999 Jul y 19. The UV-continuum flux at 1450 Angstrom decreased by factor of about 4 over the previous 2 years, and there was a corresponding dramatic increase in the column densities of the low-ionization absorption lines (e.g., Si I I, Fe II, and Al II), presumably as a result of a decrease in the ionizing continuum. In addition to the absorption lines seen in previous low states, we identify a large number of Fe II absorption lines that arise from metas table levels as high as 4.1 eV above the ground state, indicating high dens ities (>10(6) cm(-3)). We find that the transient absorption feature in the blue wing of the broad C IV emission, seen in a Goddard High Resolution Sp ectrograph spectrum and thought to be a high-velocity C IV component, is ac tually a Si II fine-structure absorption line at a radial velocity of -560 km s(-1) (relative to systemic). We also demonstrate that the "satellite" e mission lines of C IV found in IUE spectra are actually regions of unabsorb ed continuum plus broad emission that become prominent when the UV continuu m of NGC 4151 is in a low state.