Polar Dust Emission in Quasar IR SEDs and Its Correlation with Narrow Line Regions

Abstract

Polar dust has been found to play an important role in the mid-infrared emission of nearby Seyfert nuclei. If and how often polar dust exists among the quasar population is unknown due to the lack of spatially-resolved observations. In this Letter, we report correlations between the prominence of AGN forbidden line emission (commonly associated with the narrow line region) and the dust mid-IR energy output among the archetypal Palomar-Green quasar sample and other bright type-1 AGNs drawn from the SDSS, Spitzer and WISE archives. The AGN mid-IR color differences traced by WISE W2 ($\sim 4.6\mu m$)$-$W3 ($\sim 12\mu m$) and W2 ($\sim 4.6\mu m$)$-$W4 ($\sim 22\mu m$), and near-IR to mid-IR SEDs constrained with 2MASS, WISE and Spitzer data have clear trends with the relative strength of the forbidden line regions traced by the optical \OIII and mid-IR \OIV emission lines. These observations indicate that, where the lines are strong, a large fraction of the AGN emission at $\lambda \gtrsim 5\mu$m comes from dust in the forbidden line regions. We find that the widely quoted universal AGN template is a result of averaging quasar SEDs with different levels of polar dust emission above the torus output and that the typical intrinsic IR SED of compact torus dust emission alone falls with increasing wavelength past 5 $\mu$m (in $\nu F_{\nu }$). In addition, the association of polar dust with the forbidden lines suggests an alternative to the receding torus hypothesis for the decrease in infrared output with increasing AGN luminosity.

arXiv comment9 pages, 4 figures, accepted for publication in ApJ Letter

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