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

Abstract

For 15 bright (V<17.5), high redshift (z>3) quasars, we have obtained IR spectra and photometry, and optical spectrophotometry and photometry, which we use to construct their rest-frame 1285-5100 AA spectral energy distributions (SEDs). High resolution spectroscopy for 7 and L&#39; detections of 4 extend their SEDs shortwards of Ly_alpha or redwards of 7500 AA. The average luminosities within a set of narrow, line-free, windows are computed. Power law fits to those from 1285 to 5100 AA, but excluding the 2000-4000 AA region of the FeII+BaC `small bump&#39;, adequately characterize the continuum shapes of most of the objects and yield optical/UV spectral indices (aouv: fnu ~ nu^{aouv}). To look for signs of evolution, we compare the distribution of the aouv for the z>3 quasars with that for a set of 27 z~0.1 quasars which are matched to the high redshift(z) ones in evolved luminosity. The mean (median) aouv for the high and low z samples are -0.32(-0.29) and -0.38(-0.40), respectively. Neither the distributions nor means differ significantly. The distributions of aouv for both samples span a wide range however, with Delta~1, which could be understood as the result of both a diversity in the emitted continua themselves and in the amounts of intrinsic extinction undergone. A clear difference between the high and low z samples occurs in the region of `small bump&#39;. The power law fit residuals for the low z sample show a systematic excess from 2200-3000 AA ; this feature is weak or absent in the high z sample. Further study is needed, but the `small bump&#39; evolution could reflect differences in iron abundance or FeII energy source, or alternatively, an intrinsic turnover in the continuum itself which is present at low but not at high z. (Abridged)