Evolutionary Consequences of Dusty Tori in Active Galactic Nuclei

Abstract

Deep surveys by Chandra and the Hubble Space Telescope (HST) show that active galactic nucleus (AGN) populations are changing with hard X-ray luminosities. Thus, an interesting question arises as to whether or not the dusty torus is evolving with the central engines. We assemble a sample of 50 radio-quiet Palomar-Green quasars to tackle this problem. The covering factors of the dusty tori can be estimated from the multiwavelength continuum. We find that they are strongly correlated with the hard X-ray luminosity. Interestingly, this correlation agrees with the fraction of type II AGNs discovered by Chandra and HST, implying strong evidence of the changing results of the AGN population from the evolution of the tori. We also find that the frequencies of the dips at around 1 μm in the continuum correlate with the covering factors in the present sample, indicating that the dip frequencies are adjusted by the covering factors. In the scenario of the fueling of a black hole from the torus, the covering factor is good, and the dip frequency is a potential indicator of the torus evolution.