2005ApJ...619...86C

The dust clouds in the torus of a quasar are irradiated by the central source, and the clouds at the inner radius of the torus reradiate mostly in the near-infrared (NIR) wave bands. The ratio of the NIR luminosity to the bolometric luminosity, L_NIR/L_bol, may therefore reflect the torus geometry to some extent. We find a significant correlation between the ratio of the NIR luminosity to the bolometric luminosity L_NIR/L_boland the central black hole mass M_bhfor Palomar-Green (PG) quasars, whereas no correlation is found between the Eddington ratio L_bol/L_Eddand the L_NIR/L_bol. Similar correlations are found for the mid-infrared and far-infrared. This may imply that torus geometry, i.e., the solid angle subtended by the dust torus as seen from the central source, does not evolve with the accretion rate. The correlation of the solid angle subtended by the torus with the central black hole mass M_bhimplies that the formation of the dust torus is likely regulated by the central black hole mass. We find that the torus thickness H increases with quasar bolometric luminosity, which differs from the constant torus thickness H assumed in the receding torus model. The average relative thickness H/R of the tori in the PG quasars derived from the ratios of the infrared to bolometric luminosities is ∼0.9. Further infrared observations of a larger quasar sample, including fainter quasars, by the Spitzer Space Telescope will help our understanding of the physics of the dust tori in quasars.