STIS Echelle Observations of the Intrinsic UV Absorption in the Seyfert 1 Galaxy NGC 3783

Abstract

We present observations of the UV absorption lines in the Seyfert 1 galaxy NGC 3783, obtained with the medium resolution (λ/Δλ ≈ 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 Lyα, 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 ~5 yr earlier. Somewhat unexpectedly, each component has a covering factor (of the continuum source+broad emission line region) of ~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 × 1020-1.5 × 1021 cm-2) for each component, and a second, low-ionization (U = 0.0018) and low column density (4.9 × 1018 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), 912-1200 A, 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 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.