Composite Quasar Spectra from the Sloan Digital Sky Survey

We have created a variety of composite quasar spectra using a homogeneousdata set of over 2200 spectra from the Sloan Digital Sky Survey (SDSS). Thequasar sample spans a redshift range of 0.044 <= z <= 4.789, and an absolute r'magnitude range of -18.0 to -26.5. The median composite covers a restwavelength range from 800 - 8555A, and reaches a peak signal-to-noise ratio ofover 300 per 1A resolution element in the rest frame. We have identified over80 emission line features in the spectrum. Emission line peak velocity shiftsof the broad permitted and semi-forbidden lines are strongly correlated withionization energy, as previously suggested, but we find that the narrowforbidden lines are also shifted by amounts which are strongly correlated withionization energy. The magnitude of the forbidden line shifts is < 100 km/s,compared to shifts of up to 550 km/s for some of the permitted andsemi-forbidden lines. At wavelengths longer than the Ly-a emission, thecontinuum of the geometric mean composite is well-fit by two power-laws, with abreak at approximately 5000A. The frequency power law index, alpha_nu, is -0.44from 1300 - 5000A, and -2.45 redward of about 5000A. The abrupt change in slopecan be accounted for partly by host galaxy contamination at low redshift.Stellar absorption lines, including higher-order Balmer lines, seen in thecomposites suggest that young or intermediate age stars make a significantcontribution to the light of the host galaxies. An electronic table of themedian quasar template is available.Comment: 40 pages, including 11 postscript figures, accepted by AJ; for the full version of table 1, see: ftp://sdss.fnal.gov/pub/danvb/qsocomposite/sdss_qso_median.tab