Emission-line properties of z>4 quasars

High-redshift QSOs provide important opportunities for advancing our understanding of the astrophysics of galaxy formation and evolution. Although a growing number of these sources are now known to exist at redshifts beyond 4, systematic studies of the emission-line properties of these objects are quite limited. We present results of a program of high signal-to-noise spectroscopy for 44 QSOs using the Multiple Mirror Telescope Observatory and the W. M. Keck Observatory. The quasar spectra cover 11001700 Angstrom in the rest frame for sources spanning a luminosity range of similar to2 orders of magnitude. Comparisons between these data and spectra of lower redshift quasars reveal a high degree of similarity, although differences are present in the profiles and the strengths of some emission features. An examination of the luminosity dependence of the emission lines reveals evidence for a weak or absent Baldwin effect among zgreater than or similar to4 QSOs. We compare measurements for objects in our sample with results from other high-redshift surveys characterized by different selection techniques. Distributions of equivalent widths for these different ensembles are consistent with a common parent population, suggesting that our sample is not strongly biased, or in any case, subject to selection effects that are not significantly different from other surveys, including the Sloan Digital Sky Survey. Based on this comparison, we tentatively conclude that the trends identified here are representative of high z QSOs. In particular, the data bolster indications of supersolar metallicities in these luminous, high-z sources, which support scenarios that assume substantial star formation at epochs preceding or concurrent with the QSO phenomena.