Intrinsic absorption lines in Seyfert 1 galaxies. I. Ultraviolet spectra from the Hubble Space Telescope

We present a study of the intrinsic absorption lines in the ultraviolet spe ctra of Seyfert 1 galaxies. The study is based on spectra from the Hubble S pace Telescope and includes the Seyfert I galaxies observed with the Faint Object Spectrograph and Goddard High-Resolution Spectrograph at spectral re solutions of lambda/Delta lambda approximate to 1000-20,000 with good signa l-to-noise ratios. We find that the fraction of Seyfert I galaxies that sho w intrinsic absorption associated with their active nuclei is more than one -half (10 of 17), which is much higher than previous estimates (3%-10%) bas ed on IUE data. There is a one-to-one correspondence between Seyfert galaxi es that show intrinsic UV absorption and X-ray "warm absorbers," indicating that these two phenomena are related. Although our sample is not complete, we conclude that intrinsic absorption represents an important component th at needs to be integrated into our overall physical picture of active galax ies. The intrinsic UV absorption is generally characterized by high ionizat ion: C Iv and N v are seen in all 10 Seyfert galaxies with detected absorpt ion tin addition to Ly alpha), whereas Si rv is present in only four of the se Seyfert galaxies, and Mg II absorption is detected only in NGC 4151, The absorption lines are blueshifted (or in a few cases at rest) with respect to the narrow emission lines, indicating that the absorbing gas is undergoi ng net radial outflow. At high resolution, the absorption often splits into distinct kinematic components that show a wide range in widths (20-400 km s(-1) FWHM), indicating macroscopic motions (e.g., radial velocity subcompo nents or turbulence) within a component. The strong absorption components h ave cores that are much deeper than the continuum flux levels, indicating t hat the regions responsible for these components lie completely outside of the broad emission-line regions. Additional information on the covering fac tors and column densities can be derived from the absorption profiles in th e high-resolution spectra. The covering factor of the absorbing gas in the Line of sight, relative to the total underlying emission, is C-los greater than or equal to 0.86, on average. The global covering factor, which is the fraction of emission intercepted by the absorber averaged over all lines o f sight, is C-global greater than or equal to 0.5. Thus, structures coverin g large solid angles as seen by the central continuum source (e.g., spheric al shells, sheets, or cones with large opening angles) are required. The in dividual absorption components show a wide range in C Iv column densities ( 0.1-14 x 10(14) cm(-2)), and the ratio of N v to C rv column density varies significantly from one absorption component to the next, even in the same Seyfert galaxy. Thus, the intrinsic absorption in a Seyfert 1 galaxy is typ ically comprised of distinct kinematic components that are characterized by a range in physical conditions (e.g., ionization parameter and hydrogen co lumn density). Finally, we show evidence for extreme variability in the int rinsic absorption lines of NGC 3783. In addition to our earlier report of t he appearance of a C N absorption doublet at -560 km s(-1) (relative to the emission lines) over 11 months, we have detected the appearance of another C rv doublet at - 1420 km s(-1) over 15 months. On the other hand, the C I V absorption lines of NGC 3516 and NGC 4151 were very stable over periods o f 6 months and;4 years, respectively. Monitoring observations of individual Seyfert galaxies at higher time resol ution are needed to distinguish between different sources of variability (v ariable ionization, motion of gas across the line of sight) and to determin e the densities and radial locations of the absorption components.