A search for signatures of quasar evolution: Comparison of the shapes of the rest-frame optical/ultraviolet continua of quasars at z > 3 and z similar to 0.1

For 15 bright (V < 17.5), high-redshift (z > 3) quasars, we have obtained i nfrared spectra and photometry, and optical spectrophotometry and photometr y, which we use to construct their spectral energy distributions (SEDs) fro m lambda (rest) similar to 1285-5100 Angstrom. High-resolution spectroscopy for seven enable measure ments of their continua shortward of Ly alpha, an d L' detections of four of these extend their SEDs redward to lambda (rest) similar to 7500 Angstrom. We examine the optical/UV continuum shapes and c ompare these to those of a set of 27 well-studied low-redshift (z similar t o 0.1) quasars which are matched to the high-redshift ones in evolved lumin osity. Single power-law fits to the average fluxes within a set of narrow, line-free, windows between 1285 and 5100 Angstrom, but excluding the 2000-4 000 Angstrom region of the Fe II + BaC "small bump," are adequate for most of the objects. For both the high- and low-redshift samples, the distributi ons of spectral indices, alpha (ouv) (F-nu similar to nu (alpha ouv)) span a wide range, with Delta alpha (ouv) similar to 1. The cause of such divers ity is investigated, and our analysis is consistent with the conclusion of Rowan-Robinson: that it arises from differences in both the emitted continu a themselves and in the amounts of intrinsic extinction undergone. The mean (median) optical/UV spectral indices for the high- and low-redshift sample s are -0.32 (-0.29) and -0.38 (-0.40), respectively. A Student's t-test ind icates that these do not differ significantly, and a K-S test shows likewis e for the distributions. Assuming the optical/UV continuum derives from acc retion, the similarity of the spectral indices at high and low redshift is inconsistent with models which interpret the statistical evolution as resul ting from a single generation of slowly dimming quasars and instead favors those involving multiple generations of short-lived quasars formed at succe ssively lower luminosities. A clear difference between the high- and low-re dshift samples occurs in the region of "small bump." The power-law fit resi duals for the low-redshift sample show a systematic excess from similar to 2200 to 3000 Angstrom; but this feature is weak or absent in the high-redsh ift sample. Further study is needed to determine what is responsible for th is contrast, but it could reflect differences in iron abundance or Fe II en ergy source, or alternatively, an intrinsic turnover in the continuum itsel f which is present at low but not at high redshift.