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

Abstract

By combining the newly infrared photometric data from the All-Sky Data Release of the Wide-field Infrared Survey Explorer with the spectroscopic data from the Seventh Data Release of the Sloan Digital Sky Survey, we study the covering factor of warm dust ($\CF$) for a large quasar sample, as well as the relations between $\CF$ and other physical parameters of quasars. We find a strong correlation between the flux ratio of mid-infrared to near-ultraviolet and the slope of near-ultraviolet spectra, which is interpreted as the dust extinction effect. After correcting for the dust extinction utilizing the above correlation, we examine the relations between $\CF$ and AGN properties: bolometric luminosity ($\Lbol$), black hole mass ($\MBH$) and Eddington ratio ($L/L_{\rm Edd}$). We confirm the anti-correlation between $\CF$ and $\Lbol$. Further we find that $\CF$ is anti-correlated with $\MBH$, but is independent of $L/L_{\rm Edd}$. Radio-loud quasars are found to follow the same correlations as for radio-quiet quasars. Monte Carlo simulations show that the anisotropy of UV-optical continuum of the accretion disc can significantly affect, but is less likely to dominate the $\CF$--$\Lbol$ correlation.