The Hubble Space Telescope sample of radio-loud quasars: The Lyα/Hβ ratio

We have used the first Hubble Space Telescope Faint Object Spectrograph spectra of our sample of radioloud quasars, and quasi-simultaneous ground-based spectrophotometry, to investigate the intensity ratio Lyα/ Hβ, whose small observed values are one of the outstanding problems of active galactic nuclei research. The present sample of 20 quasars with complete flux and profile data shows the first significant correlations of this ratio with other observed properties. The strongest correlations are with various continuum slope indicators: we find smaller Lyα/Hβ ratios in quasars whose continua rise more steeply into the red. The long-wavelength continuum slope (1909-4861 Å) is strongly correlated with Lyα/Hβ, but the short-wavelength continuum slope (1215-1909 Å) is not. A separation into line components shows that the above correlations arise mostly from the red wings of the lines. The core-to-wing flux ratio is also correlated with the slope. The correlation of Lyα/Hβ with continuum slope is consistent with line and continuum reddening by an external dust screen with Galactic-type extinction of up to E_B-V = 0.3. In this case the intrinsic Lyα/Hβ ratio is ∼20. However, other trends expected if dust were the sole factor are not seen. There are indications that core-dominated and lobe-dominated sources differ in their Lyα/Hβ and continuum slope dependence. We calculate a grid of theoretical hydrogen line ratios and use it to investigate reddening and alternative explanations, such as dependence upon ionizing flux. We suggest that several different mechanisms are operating.