Improved redshifts for SDSS quasar spectra

A systematic investigation of the relationship between different redshift estimation schemes for more than 91 000 quasars in the Sloan Digital Sky Survey (SDSS) Data Release 6 is presented. The publicly available SDSS quasar redshifts are shown to possess systematic biases of Delta z/(1 + z) >= 0.002 (600 km s(-1)) over both small (delta z similar or equal to 0.1) and large (delta z similar or equal to 1) redshift intervals. Empirical relationships between redshifts based on (i) Ca II H&K host galaxy absorption, (ii) quasar [O II] lambda 3728, (iii) [O III] lambda lambda 4960, 5008 emission and (iv) cross-correlation (with a master-quasar template) that includes, at increasing quasar redshift, the prominent Mg II lambda 2799, C III] lambda 1908 and C IV lambda 1549 emission lines are established as a function of quasar redshift and luminosity. New redshifts in the resulting catalogue possess systematic biases, a factor of similar or equal to 20 lower compared to the SDSS redshift values; systematic effects are reduced to the level of Delta z/(1 + z) <= 10(-4) (30 km s(-1)) per unit redshift or <= 2.5 x 10(-5) per unit absolute magnitude. Redshift errors, including components due both to internal reproducibility and to the intrinsic quasar-to-quasar variation among the population, are available for all quasars in the catalogue. The improved redshifts and their associated errors have wide applicability in areas such as quasar absorption outflows, quasar clustering, quasar-galaxy clustering and proximity-effect determinations.