New constraints on quasar evolution: broad-line velocity shifts over 1.5 ≲ z ≲ 7.5

ABSTRACT We present the results of a model-independent investigation of the rest-frame UV spectra from a comprehensive sample of 394 quasars in the redshift range 1.5 ≤ z ≤ 7.5. We fit the main broad emission lines (BELs) in the rest-frame range 1280 Å ≤ λ ≤ 3000 Å (O i, C ii, Si iv, C iii], C iv, and Mg ii) with a lightly supervised spline fitting technique. Redshifts are derived from the peaks of each fitted BEL and used to compute relative velocity shifts. We show that our method gives unbiased velocity shifts and is insensitive to spectral resolution and instrumental parameters. It is found that the average blueshift of the C iv line with respect to several low-ionization lines in luminosity-matched samples does not significantly evolve over 1.5 ≤ z ≤ 6. However, the average blueshift increases significantly by a factor of ∼2.5 at z ≳ 6. We propose that this redshift evolution can be explained by C iv winds launched perpendicularly to an accretion disc with increased torus opacity at high redshift, coupled with a potential orientation-driven selection bias. Our results open new exciting avenues of investigation into young quasars in the reionization epoch.