H-alpha emission lines in high-redshift quasars

We have obtained, using the FIGS spectrometer at the Anglo-Australian Telescope, infrared spectra of the Hα lines in 18 medium- to high-redshift QSOs and optical spectra taken nearly simultaneously to measure the strong ultraviolet lines. We find that the Hα line is redshifted by an average of 1000 km s^-1^ with respect to the lines from high ionization species such as C IV. Low ionization lines, from ions such as O I and Mg II, are shifted by similar, or slightly smaller, amounts with respect to the high ionization lines. These results are difficult to reconcile with any simple models currently available, including those where dust obscuration is solely responsible for the observed velocity shifts. The similarity between the velocities of Hα and Mg II, O I provides some support for models in which the Balmer lines are produced predominantly in a warm H I region, perhaps by X-ray heating, while the Lyman lines arise mainly in a population of optically thin clouds. A velocity separation between the two cloud populations, along with some obscuration, could explain the main features. However, detailed differences between lines of similar ionization suggest that the true situation is more complex. A search for correlations between the velocity shift and other parameters revealed only one, with the ratio of the C III] 1909 (A) to C IV 1549 line fluxes. The meaning of this is obscure, but may relate to density or projection effects. A number of other correlations proposed by previous workers as tests for their models were sought but not found.