2013ApJ...773..176G

We have assembled a large sample of 5996 quasars at 2.0 ≤ z ≤ 2.4 (high-z) or 0.7 ≤ z ≤ 1.1 (low-z) from the Sloan Digital Sky Survey (SDSS) ninth and seventh data release and quasar catalogs. The spectral energy distributions of quasars were constructed by collecting WISE, UKIDSS, and GALEX photometric data in addition to SDSS data, from which the IR luminosity at 1-7 µm and bolometric luminosity at 1100 Å-1 µm were calculated. A red tail is clearly seen in the distribution of the spectral index over 1100 Å-1 µm for both the high-z and low-z sources; this tail is likely due to red or reddened quasars. The covering factor (CF) of the dusty torus is estimated as the ratio of the IR luminosity to the bolometric luminosity. We find significant anti-correlations between the CF and the bolometric luminosity, in both the high-z and low-z quasars; however, these two groups follow different tracks. At overlapping bolometric luminosities, the CF of high-z quasars is systematically larger than those of low-z quasars, implying an evolution of the CF with redshift.