Hubble Space Telescope Sample of Radio-loud Quasars: Ultraviolet Spectra of the First 31 Quasars

We report the first results from a continuing program to investigate the multifrequency spectrophotometric and other properties of a sample of about 50 radio-loud quasars in the redshift range ∼0.3-1.3. Here we present spectrophotometric data of high signal-to-noise ratio (≳20 in the continuum) of the first 31 radio-loud quasars, over the wavelength range from below Lyman alpha to an observed wavelength of 3250 or 4800 Å, obtained using the Faint Object Spectrograph on the Hubble Space Telescope. We have also made quasi-simultaneous observations to extend these spectra beyond the Balmer lines -- either Hβ or Hα -- and those data will be presented in companion papers. In the ultraviolet and optical, the resolution is 300-400 km s-1 and in the infrared, about 1000 km s-1. The sample was selected by luminosity of the extended radio emission, and to cover a range of radio core-dominance, in order to test ideas relating to orientation of the continuum and emission- line regions to the observer's line of sight. We present the ultraviolet spectra, and tabulate basic spectroscopic measurements: continuum flux densities and spectral indices, intensities and equivalent widths for all strong emission lines after deblending, as well as profile parameters such as various line widths and asymmetries. The relations between these and other parameters will be presented in detail in a later paper. In our low-redshift sample we have good coverage from C III λ977 to O VI λ1035, a region that is very confused by Lyman forest absorption in high-redshift quasars observed from the ground. We find that Lyβ is less than 10% of the strength of O VI λ1035. The ratio O VI λ1035/Lyα ∼0.21 supports photoionization models with high ionization and a small, dense broad-line region, although the weakness of C III λ977 typically 5% of Lyα-limits the amount of gas with densities ≳ 1010-1011 cm-3. The N III λ991 line is present at about 4% the strength of Lyα. In contrast to the lobe-dominant quasars, which have flat ultraviolet-optical continua (in Fv) all quasars with steep continua are radio core-dominant, and their broad lines tend to have lower equivalent width. Associated absorption is more prominent in the lobe-dominant quasars. Both these results are consistent with unified schemes in which a synchrotron-emitting jet is viewed at small angles to the line-of-sight in core-dominant quasars, and the line-of-sight at larger angles passes through cooler gas. In addition, we find and discuss significant differences in C IV λ1549 line asymmetries: the profiles of core-dominant quasars have stronger red wings, profiles of lobe-dominant quasars tend to be symmetric with both red and blue wings, and radio-quiet QSO profiles have stronger blue wings.