Dust-deficient Palomar-Green Quasars and the Diversity of AGN Intrinsic IR Emission

Abstract

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 µm 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 AGNs 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.

The Intrinsic Far-infrared Continua of Type-1 Quasars

“…Since Elvis et al (1994) published their SED template for unobscured type-1 (broad-line) quasars, there have been multiple studies that have confirmed the uncanny accuracy of this work (e.g., Richards et al 2006;Shang et al 2011;Runnoe et al 2012;Hanish et al 2013;Scott & Stewart 2014;Lyu et al 2017). As a result, we can take the intrinsic SED of a type-1 quasar to be well-understood from the soft X-ray to ∼10 μm.…”

“…However, as suggested in Section 3.1 of Lyu et al (2017), the host contribution in the infrared 8-1000 μm range could be underestimated by merely focusing on the mid-IR spectral features. This argument is based on the different IR SED behaviors of the AGN and the host galaxy: the IR output of an AGN is peaked in the mid-IR but drops quickly in the far-IR, while the galaxy emission is relatively weak in the mid-IR but strong in the far-IR.…”

“…As shown by our previous work (Lyu et al 2017), it is clear that one single AGN IR template does not apply to all the type-1 quasar population (   L L 10…”

“…We illustrate some issues in deriving intrinsic AGN IR SEDs by extending the discussion in Lyu et al (2017) on the infrared SEDs of PG quasars.…”

“…First, an energy balance argument is applied to test some proposed AGN SED templates in Section 2. Then we present an in-depth analysis of the far-IR emission of the Palomar-Green (PG) quasars based on the study of Lyu et al (2017), clarify several issues regarding the derivation of a correct intrinsic AGN far-IR SED, and determine our preferred template in Section 3. In Section 4, we discuss whether the AGN far-IR intrinsic template based on quasars is applicable to other kinds of AGNs, as well as some implications for the obscuring structure from our results.…”