Spectral evolution in high redshift quasars from the final boss sample

We report on a study of the spectral variations in a sample of 102,150 quasars from the Baryon Oscillation Spectroscopic Survey (BOSS) of the Sloan Digital Sky Survey (SDSS-III). After mitigating selection effects and Malmquist bias over the redshift range 2:1 □ z □ 3:5, we create high signal-to-noise composite spectra binned by luminosity, spectral index, and redshift. We use these composite spectra to inspect the variations in quasar properties as a function of each of these three parameters. We confirm the traditional Bald­ win effect (i.e. the anti-correlation of C IV equivalent width and luminosity) that follows the relation W |j Lbw with slope bw = D0:80 □ 0:005;D0:76 □ 0:006; mid □ 0:88 □ 0:006 for z= 2:26; 2:46; rnd 2:84 respectively. We also observe for a unit change in the spectral index, redder quasars exhibit an apparent outflow in C IV with a velocity 3100 □ 80 kms~1 relative to the estimated systemic redshift. Finally, we find evidence for redshift evolution in average quasar spectral features, even at a fixed luminosity. This evolution has a signa­ ture nearly identical to the signature of the Baldwin effect. The two trends are correlated at 94%, indicating that they arise from the same physical mechanism. We argue that this spectral variation is likely the signature of varying accretion efficiency.