Dust-Deficient Palomar-Green Quasars and the Diversity of AGN Intrinsic
  IR Emission
  To elucidate the intrinsic broadband infrared (IR) emission properties of
active galactic nuclei (AGNs), we analyze the spectral energy distributions
(SEDs) of 87 z<0.5 Palomar-Green (PG) quasars. While the Elvis AGN template
with a moderate far-IR correction can reasonably match the SEDs of the AGN
components in ~60% of the sample (and is superior to alternatives such as that
by Assef), it fails on two quasar populations: 1) hot-dust-deficient (HDD)
quasars that show very weak emission thoroughly from the near-IR to the far-IR,
and 2) warm-dust-deficient (WDD) quasars that have similar hot dust emission as
normal quasars but are relatively faint in the mid- and far-IR. After building
composite AGN templates for these dust-deficient quasars, we successfully fit
the 0.3-500 {\mu}m SEDs of the PG sample with the appropriate AGN template, an
infrared template of a star-forming galaxy, and a host galaxy stellar template.
20 HDD and 12 WDD quasars are identified from the SED decomposition, including
seven ambiguous cases. Compared with normal quasars, the HDD quasars have AGN
with relatively low Eddington ratios and the fraction of WDD quasars increases
with AGN luminosity. Moreover, both the HDD and WDD quasar populations show
relatively stronger mid-IR silicate emission. Virtually identical SED
properties are also found in some quasars from z = 0.5 to 6. We propose a
conceptual model to demonstrate that the observed dust deficiency of quasars
can result from a change of structures of the circumnuclear tori that can occur
at any cosmic epoch.
