Intrinsic absorption lines in the Seyfert 1 galaxy NGC 5548: Ultraviolet echelle spectra from the space telescope imaging spectrograph

We present the first observations of a Seyfert galaxy with the echelle grat ings on the Space Telescope Imaging Spectrograph (STIS), which provide high -resolution (lambda/Delta lambda approximate to 40,000) coverage of the int rinsic UV absorption lines in NGC 5548. We confirm the presence of five kin ematic components of absorption in Ly alpha, C IV, and N V at radial veloci ties of -160 to -1060 km s(-1) with respect to the emission lines, and find an additional Lya component near the systemic velocity, which probably ari ses in the interstellar medium of the host galaxy. Compared to Goddard High -Resolution Spectrograph spectra of the N V and C IV absorption obtained si milar to 2 yr 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 f or one component we cannot rule out changes in the ionization of the gas. W e have calculated photoionization models to match the UV column densities f rom each of the five components associated with the nucleus. In four of the components, the ionization parameters (U = 0.15-0.80) and effective hydrog en column densities (N-eff = 6.0 x 10(18) to 2.8 x 10(20) cm(-2)) cannot pr oduce 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-eff = 6.5 x 10 (21) cm(-2)), and our model matches the previously observed X-ray absorptio n columns. This component is therefore likely to be responsible for the X-r ay-warm absorber. It also has the highest outflow velocity and showed the l argest variations in column density.