Intrinsic Absorption Lines in the Seyfert 1 Galaxy NGC 5548: UV Echelle Spectra from the Space Telescope Imaging Spectrograph

Abstract

We present the first observations of a Seyfert galaxy with the echelle gratings on the Space Telescope Imaging Spectrograph (STIS), which provide high-resolution (R = 40,000) coverage of the intrinsic UV absorption lines in NGC 5548. We confirm the presence of five kinematic components of absorption in Lyman-alpha, C IV, and N V at radial velocities of -160 to -1060 km/s with respect to the emission lines, and find an additional Lyman-alpha$ component near the systemic velocity, which probably arises in the interstellar medium of the host galaxy. Compared to GHRS spectra of the N V and C IV absorption obtained ~2 years earlier, the kinematic components have not changed in radial velocity, but the ionic column densities for two components have decreased. We attribute these variations to changes in the total column of gas, but for one component, we cannot rule out changes in the ionization of the gas. We have calculated photoionization models to match the UV column densities from each of the five components associated with the nucleus. In four of the components, the ionization parameters (U = 0.15 - 0.80) and effective hydrogen column densities (N = 6.0 x 10^18 cm^-2 -- 2.8 x 10^20 cm^-2) cannot produce the O VII and O VIII absorption edges seen in the X-ray warm absorber. The remaining component is more highly ionized (U = 2.4, N = 6.5 x 10^21 cm^-2) and our model matches the previously observed X-ray absorption columns. This component is therefore likely to be responsible for the X-ray warm absorber. It also has the highest outflow velocity and showed the largest variations in column density.