Combined ultraviolet and optical spectra of 48 low-redshift QSOs and the relation of the continuum and emission-line properties

We present combined ultraviolet and optical spectra of 48 QSOs and Seyfert 1 galaxies in the redshift range 0.034-0.774. The UV spectra were obtained nonsimultaneously with the optical and are derived from archival Hubble Space Telescope (HST) Faint Object Spectrograph and International Ultraviolet Explorer (IUE) observations. The sample consists of 22 radio-quiet objects, 12 fiat radio spectrum radio-loud objects, and 14 steep radio spectrum objects, and it covers approximately 2.5 decades in ultraviolet continuum luminosity. The sample objects are among the most luminous known in this redshift range and include 3C 273 and Fairall 9, as well as many objects discovered in the Bright Quasar Survey. We measure and compare an array of emission-line and continuum parameters, including 2 keV X-ray luminosities derived from the Einstein database. We examine individual correlations and also apply a principal components analysis (PCA) in an effort to determine the underlying sources of variance among these observables. Our main results are as follows. 1. The C IV lambda 1549 profile asymmetry is correlated with the UV continuum luminosity measured at the position of that line, such that increasing continuum luminosity produces increasing redward asymmetry. This is the same correlation found between H beta asymmetry and 2 keV luminosity in a larger sample of objects and appears to be followed by both radio-loud and radio-quiet sources. The C IV profile asymmetry is also correlated with the FWZI of the Ly alpha profile, with more redward asymmetric profiles associated with wider profile bases. The PCA reveals that the correlated increase in luminosity, C IV redward asymmetry, and profile base width accounts for over half the statistical variance in the sample. 2. There is a statistically significant difference between the FWZI distributions of the Ly alpha and H beta lines, such that the former is wider on average by similar to 10(4) km s(-1). The FWHM values of the broad H beta line are weakly correlated with those of C IV lambda 1549 and Ly alpha, and in contrast to the FWZI values the H beta profiles are wider. Measures of the asymmetry of the H beta and C IV profiles also show a weak correlation. The wavelength centroids at 3/4 maximum of the Ly alpha and C IV lines also show average blueshifts similar to 50-200 km s(-1) from [O III] lambda 5007, versus an average redshift of 75 km s(-1) for broad H beta. 3. There is no clear evidence of narrow components to the stronger UV lines, even among objects in which the optical narrow lines including [O III] lambda lambda 4959, 5007 are unusually strong. We measure the average fractional contributions of such components to the Ly alpha and [C III] lambda 1909 lines to be similar to 4%-5%, consistent with the findings from smaller samples. However, a sizable fraction (similar to 50%) of radio-loud objects display a narrow component of He II lambda 1640, the same as in the QSO population at intermediate redshifts, and such a component is likely to contribute to the other UV lines. We interpret the first result as the effect of a black hole mass/luminosity relation in which the profile widths and redward asymmetries are produced respectively by the virialized motions and gravitational redshift associated with 10(9)-10(10) M. holes. This does not explain the cases of blueward profile asymmetries and blueshifted profile peaks, which require an effect acting oppositely to gravitational redshift. The peak redshift differences and relative weakness of the correlations between the UV profile widths and asymmetries and those of H beta suggests a stratified ionization structure of the broad-line region (BLR), consistent with the variability studies of Seyfert 1 galaxies. Continuum variability and the dynamical evolution of the BLR gas may also influence these results. The difference between the Ly alpha and H beta FWZI values provides additional evidence of an optically thin very broad line region (VBLR) lying interior to an intermediate line region (ILR) producing the profile cores. The smaller average FWHM values of the UV lines compared to H beta indicate that they have a higher relative contribution of ILR emission, versus a more dominant VBLR component in the Balmer lines. The narrow He II lambda 1640 feature of radio-loud objects is likely associated with the inner regions of extended (similar to 100 kpc) ionized halos that are not present around radio-quiet objects, and which appear to be best explained as cooling hows around the QSO host galaxies.