Composite Quasar Spectra from the Sloan Digital Sky Survey

Abstract

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). The quasar sample spans a redshift range of 0.044 < 100 km/s, compared to shifts of up to 550 km/s for some of the permitted and semi-forbidden lines. At wavelengths longer than the Ly-a emission, the continuum of the geometric mean composite is well-fit by two power-laws, with a break at approximately 5000A. The frequency power law index, alpha_nu, is -0.44 from 1300 - 5000A, and -2.45 redward of about 5000A. The abrupt change in slope can be accounted for partly by host galaxy contamination 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. An electronic table of the median quasar template is available.