THE ULTRAVIOLET EMISSION PROPERTIES OF 5 LOW-REDSHIFT ACTIVE GALACTIC NUCLEI AT HIGH SIGNAL-TO-NOISE RATIO AND SPECTRAL RESOLUTION

We analyze the ultraviolet (UV) emission line and continuum properties of five low-redshift active galactic nuclei (four luminous quasars: PKS 0405 - 123, H1821 + 643, PG 0953 + 414, and 3C 273, and one bright Seyfert 1 galaxy: Mrk 205). The HST spectra have higher signal-to-noise ratios (typically approximately 60 per resolution element) and spectral resolution (R = 1300) than all previously published UV spectra used to study the emission characteristics of active galactic nuclei. We include in the analysis ground-based optical spectra covering Hbeta and the narrow [O III] lambdalambda4959, 5007 doublet. The following new results are obtained: Lybeta/Lyalpha = 0.03-0.12 for the four quasars, which is the first accurate measurement of the long-predicted Lybeta intensity in QSOs. The cores of Lyalpha and C IV are symmetric to an accuracy of better than 2.5% within about 2000 km s-1 of the line peak. This high degree of symmetry of Lyalpha argues against models in which the broad line cloud velocity field has a significant radial component. The observed smoothness of the Lyalpha and C IV line profiles requires at least approximately 10(4) individual clouds if bulk velocity is the only line-broadening mechanism. The overall similarity of the Lyalpha and C IV lambda1549 profiles rules out models for the broad-line region (BLR) with a radial distribution of virialized clouds having an ionization parameter U is-proportional-to Radius-1. The measured high values of O VI lambda1034/Lyalpha and low values of C III lambda977/O VI lambda1034 imply a BLR component with U approximately 1. The excess red-wing flux in O VI relative to Lyalpha suggests the presence of an inner, high-velocity, optically thin component with U > 1 in the BLR. The N v/Lyalpha ratio is 0.135 +/- 0.01 for the four quasars, which may be an indication of higher than solar N abundance and metallicity. The maximum contribution of a narrow ([O III]-like) component is about 3%-6% of the total broad-line flux; this limit is generally highest for C III]. This result constrains the covering factor of the narrow line region or indicates the presence of dust. An unresolved component having full width at half-maximum less than 230 km s-1 typically contributes less than 0.5% of the observed broad lines flux. The HST data permit the first relatively accurate measurements of the Lygamma, C III lambda977, S VI lambdalambda933, 945, and the N III lambda991 emission lines, as well as the measurement of a number of other weak or strongly blended lines at lambda > 1216 angstrom. In agreement with observations of high-redshift quasars, the peaks of Lyalpha, C IV, and C III] are blueshifted by approximately 200 km s-1 relative to [O III] lambda5007, while He II lambda1640 is shifted by about 500 km s-1. The low ionization lines, Mg II, Hbeta, and O I lambda1304, are in most cases only marginally shifted to the red.