Spectral Evolution in High Redshift Quasars from the Final BOSS Sample
  We report on the diversity in quasar spectra from the Baryon Oscillation
Spectroscopic Survey. After filtering the spectra to mitigate selection effects
and Malmquist bias associated with a nearly flux-limited sample, we create high
signal-to-noise ratio composite spectra from 58,656 quasars (2.1 \le z \le
3.5), binned by luminosity, spectral index, and redshift. With these composite
spectra, we confirm the traditional Baldwin effect (BE, i.e., the
anticorrelation of C IV equivalent width (EW) and luminosity) that follows the
relation W_\lambda \propto L^{\beta_w} with slope \beta_w = -0.35 \pm 0.004,
-0.35 \pm 0.005, and -0.41 \pm 0.005 for z = 2.25, 2.46, and 2.84,
respectively. In addition to the redshift evolution in the slope of the BE, we
find redshift evolution in average quasar spectral features at fixed
luminosity. The spectroscopic signature of the redshift evolution is correlated
at 98% with the signature of varying luminosity, indicating that they arise
from the same physical mechanism. At a fixed luminosity, the average C IV FWHM
decreases with increasing redshift and is anti-correlated with C IV EW. The
spectroscopic signature associated with C IV FWHM suggests that the trends in
luminosity and redshift are likely caused by a superposition of effects that
are related to black hole mass and Eddington ratio. The redshift evolution is
the consequence of a changing balance between these two quantities as quasars
evolve toward a population with lower typical accretion rates at a given black
hole mass.
