We present echelle observations of the intrinsic UV absorption lines in the Seyfert galaxy NGC 4151 , which were obtained with the Space Telescope Imaging Spectrograph ( STIS ) on the Hubble Space Telescope ( HST ) on 1999 July 19 .
The UV continuum flux at 1450 Å decreased by factor of about four over the previous two years and there was a corresponding dramatic increase in the column densities of the low-ionization absorption lines ( e.g. , Si II , 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 metastable levels as high as 4.1 eV above the ground state , indicating high densities ( > 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 Spectrograph ( GHRS ) spectrum and thought to be a high-velocity C IV component , is actually 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 ” emission lines of C IV found in International Ultraviolet Explorer ( IUE ) spectra are actually regions of unabsorbed continuum plus broad emission that become prominent when the UV continuum of NGC 4151 is in a low state .