The covering factor of warm dust in quasars: a view from the Wide-field Infrared Survey Explorer All-Sky Data Release

By combining newly obtained infrared photometric data from the All-Sky Data Release of the Wide-field Infrared Survey Explorer with spectroscopic data from the Seventh Data Release of the Sloan Digital Sky Survey, we study the covering factor of warm dust (CF_WD) for a large quasar sample, as well as the relations between CF_WD and other physical parameters of quasars. We find a strong correlation between the flux ratio in the mid-infrared to near-ultraviolet and the slope of the near-ultraviolet spectra, which is interpreted as a dust extinction effect. After correcting for dust extinction utilizing the above correlation, we examine the relations between CF_WD and active galactic nucleus properties: bolometric luminosity (L_bol), black hole mass (M_BH) and Eddington ratio (L/L_Edd). We confirm the anticorrelation between CF_WD and L_bol. Further, we find that CF_WD is anticorrelated with M_BH but is independent of L/L_Edd. Radio-loud quasars are found to follow the same correlations as radio-quiet quasars. Monte Carlo simulations show that the anisotropy of the UV-optical continuum of the accretion disc can have a significant effect, but is not likely to dominate the CF_WD-L_bol correlation.