Composite quasar spectra from the Sloan Digital Sky Survey

We have created a variety of composite quasar spectra using a homogeneous d ata set of over 2200 spectra from the Sloan Digital Sky Survey (SDSS). The quasar sample spans a redshift range of 0.044 less than or equal to z less than or equal to 4.789 and an absolute r' magnitude range of -18.0 to -26.5 . The input spectra cover an observed wavelength range of 3800-9200 Angstro m at a resolution of 1800. The median composite covers a rest-wavelength ra nge from 800 to 8555 Angstrom and reaches a peak signal-to-noise ratio of o ver 300 per 1 Angstrom resolution element in the rest frame. We have identi fied over 80 emission-line features in the spectrum. Emission-line shifts r elative to nominal laboratory wavelengths are seen for many of the ionic sp ecies. Peak shifts of the broad permitted and semiforbidden lines are stron gly correlated with ionization energy, as previously suggested, but we find that the narrow forbidden lines are also shifted by amounts that are stron gly correlated with ionization energy. The magnitude of the forbidden line shifts is less than or similar to 100 km s(-1), compared with shifts of up to 550 km s(-1) for some of the permitted and semiforbidden lines. At wavel engths longer than the Ly alpha emission, the continuum of the geometric me an composite is well fitted by two power laws, with a break at approximate to 5000 Angstrom. The frequency power-law index, alpha (v), is -0.44 from a pproximate to 1300 to 5000 and -2.45 redward of approximate to 5000 The abr upt change in slope can be accounted for partly by host-galaxy contaminatio n at low redshift. 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. Most of the spectrum is populated by blended emission lines, especially in the rang e 1500-3500 Angstrom, which can make the estimation of quasar continua high ly uncertain unless large ranges in wavelength are observed. An electronic table of the median quasar template is available.