We present observations of the UV absorption lines in the Seyfert 1 galaxy NGC 3783 , obtained with the medium resolution ( \lambda / \Delta \lambda \approx 40,000 ) echelle gratings of the Space Telescope Imaging Spectrograph ( STIS ) on the Hubble Space Telescope ( HST ) .
The spectra reveal the presence of three kinematic components of absorption in Ly \alpha , C IV , and N V , at radial velocities of - 1365 , - 548 , and - 724 km s ^ { -1 } with respect to the systemic velocity of the host galaxy ( Components 1 , 2 and 3 , respectively ) ; Component 1 also shows absorption by Si IV .
Component 3 was not detected in any of the earlier Goddard High Resolution Spectrograph ( GHRS ) spectra , and the C IV absorption in the other components has changed since the most recent GHRS observation obtained \sim 5 yr earlier .
Somewhat unexpectedly , each component has a covering factor ( of the continuum source + broad emission line region ) of \sim 0.6 .
We have calculated photoionization models to match the UV column densities in each of the three components .
The models predict a zone characterized by high ionization parameter ( U = 0.65 – 0.80 ) and column density ( 6.4 x 10 ^ { 20 } – 1.5 x 10 ^ { 21 } cm ^ { -2 } ) for each component , and a second , low ionization ( U = 0.0018 ) and low column density ( 4.9 x 10 ^ { 18 } cm ^ { -2 } ) zone for Component 1 .
Based on the model results , there should be strong absorption in the bandpass of the Far Ultraviolet Spectroscopic Explorer ( FUSE ) , 912 Å – 1200 Å , including saturated O VI lines at each component velocity .
The models also predict large O VII and O VIII column densities but suggest that the UV absorbers can not account for all of the X-ray absorption detected in recent Chandra spectra .
Finally , there is no evidence for a correlation between the characteristics of the UV absorbers and the UV continuum flux , and , by inference , the ionizing continuum .
Hence , we suggest that the variations observed in the GHRS and STIS spectra are due in a large part to changes in the column densities of the absorbers as the result of transverse motion .