STIS echelle observations of the intrinsic UV absorption in the Seyfert 1 galaxy NGC 3783

We present observations of the UV absorption lines in the Seyfert 1 galaxy NGC 3783, obtained with the medium resolution (lambda/Delta lambda approxim ate to 40,000) echelle gratings of the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST). The spectra reveal the presenc e of three kinematic components of absorption in Ly alpha, C IV, and N V, a t radial velocities of -1365, -548, and -724 km s(-1) with respect to the s ystemic 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, an d the C IV absorption in the other components has changed since the most re cent GHRS observation obtained similar to5 yr earlier. Somewhat unexpectedl y, each component has a covering factor (of the continuum source+broad emis sion line region) of similar to0.6. We have calculated photoionization mode ls to match the UV column densities in each of the three components. The mo dels 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 componen t, 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 shou ld be strong absorption in the bandpass of the Far Ultraviolet Spectroscopi c Explorer (FUSE), 912-1200 Angstrom, including saturated O VI lines at eac h component velocity. The models also predict large O VII and O VIII column densities, but suggest that the UV absorbers cannot 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.