Most of the spectrum is populated by blended emission lines, especially in the range 1500-3500 Å, which can make the estimation of quasar continua highly uncertain unless large ranges in wavelength are observed. Stellar absorption lines, including higher order Balmer lines, seen in the composites suggest that young or intermediate-age stars make a significant contribution to the light of the host galaxies. The abrupt change in slope can be accounted for partly by host-galaxy contamination at low redshift. At wavelengths longer than the Lyα emission, the continuum of the geometric mean composite is well fitted by two power laws, with a break at ≈5000 Å. The magnitude of the forbidden line shifts is ≲100 km s -1, compared with shifts of up to 550 km s -1 for some of the permitted and semiforbidden lines. Peak shifts of the broad permitted and semiforbidden lines are strongly correlated with ionization energy, as previously suggested, but we find that the narrow forbidden lines are also shifted by amounts that are strongly correlated with ionization energy. Emission-line shifts relative to nominal laboratory wavelengths are seen for many of the ionic species. We have identified over 80 emission-line features in the spectrum. The median composite covers a rest-wavelength range from 800 to 8555 Å and reaches a peak signal-to-noise ratio of over 300 per 1 Å resolution element in the rest frame. The input spectra cover an observed wavelength range of 3800-9200 Å at a resolution of 1800. The quasar sample spans a redshift range of 0.044 ≤ z ≤ 4.789 and an absolute r′ magnitude range of -18.0 to -26.5. We have created a variety of composite quasar spectra using a homogeneous data set of over 2200 spectra from the Sloan Digital Sky Survey (SDSS). Szokoly, Christy Tremonti, Alan Uomoto, Patrick Waddell, Brian Yanny, Wei Zheng

Siegmund, Stephen Smee, Yehuda Snir, Chris Stoughton, Christopher Stubbs, Mark Subbarao, Alexander S. Lupton, Avery Meiksin, Aronne Merelli, Jeffrey A. Hennessy, Željko Ivezić, Stephen Kent, Peter Z. Crocker, István Csabai, Mamoru Doi, Douglas Finkbeiner, Scott Friedman, Joshua A. Brinkmann, Robert Brunner, Scott Burles, Larry Carey, Francisco J. 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.Daniel E. 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. New redshifts in the resulting catalogue possess systematic biases, a factor of 20 lower compared to the SDSS redshift values systematic effects are reduced to the level of (30kms -1) per unit redshift or per unit absolute magnitude.

Empirical relationships between redshifts based on (i) CaII H&K host galaxy absorption, (ii) quasar 3728, (iii) 4960, 5008 emission and (iv) cross-correlation (with a master-quasar template) that includes, at increasing quasar redshift, the prominent MgII2799, CIII]1908 and CIV1549 emission lines are established as a function of quasar redshift and luminosity. The publicly available SDSS quasar redshifts are shown to possess systematic biases of (600kms -1) over both small and large redshift intervals. Abstract : A systematic investigation of the relationship between different redshift estimation schemes for more than 91000 quasars in the Sloan Digital Sky Survey (SDSS) Data Release 6 is presented.

Quasar spectra