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Mass Outflow in the Seyfert 1 Galaxy NGC 5548

We present a study of the intrinsic UV absorption and emission lines in an historically low-state spectrum of the Seyfert 1 galaxy NGC 5548, which we obtained in 2004 February at high spatial and spectral resolution with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope. We isolate a component of emission with a width of 680 km/s (FWHM) that arises from an "intermediate line region" (ILR), similar to the one we discovered in NGC 4151, at a distance of ~1 pc from the central continuum source. From a detailed analysis of the five intrinsic absorption components in NGC 5548 and their behavior over a span of 8 years, we present evidence that most of the UV absorbers only partially cover the ILR and do not cover an extended region of UV continuum emission, most likely from hot stars in the circumnuclear region. We also find that four of the UV absorbers are at much greater distances (>70 pc) than the ILR, and none have sufficient N V or C IV column densities to be the ILR in absorption. At least a portion of the UV absorption component 3, at a radial velocity of -530 km/s, is likely responsible for most of the X-ray absorption, at a distance < 7 pc from the central source. The fact that we see the ILR in absorption in NGC 4151 and not in NGC 5548 suggests that the ILR is located at a relatively large polar angle (~45 degrees) with respect to the narrow-line region outflow axis.