Dust Deficient Palomar-Green Quasars and Their High-redshift Counterparts

The Elvis 1994 AGN template has been phenomenally successful at matching the SEDs of the vast majority of Type-1 quasars from the local Universe to z~6. Nonetheless, a population of hot-dust-poor or hot-dust-free quasars has been found both at intermediate and high-z. Estimates of the numbers of these anomalous quasars vary widely, along with speculations on their place in quasar evolution, e.g., whether they are related to the early growth of the black holes. To answer some of these questions, we have decomposed the optical-IR SEDs of 87 local Palomar-Green (PG) quasars: we find ~20% of them show only weak hot dust emission, indicating the hot dust deficient (HDD) quasars may be relatively common. Moreover, there is an intermediate population of warm dust deficient (WDD) quasars. Based on the comparison between the IR SEDs and classical torus models, the hot-dust-deficient quasars may have relatively little flaring in the dust density distribution in their circumnuclear tori. Our SED decomposition also indicates that the dust deficient quasars are more likely to show lower Eddington ratios and weaker star formation than the majority of quasars. Given these systematic differences, the HDD, WDD, and normal quasars could indicate a sequence of torus evolution around the accreting super-massive black holes.