On the Anisotropy of Nuclei Mid-Infrared Radiation in Nearby Active Galactic Nuclei

Abstract

In the center of active galactic nuclei (AGN), the dusty torus absorb the radiation from the central engine and re-emit in mid-infrared (MIR). Observations have detected moderate anisotropy in the dust MIR emission, in the way that type 1 AGNs (type1s) are mildly brighter in MIR comparing with type 2 sources (type2s). However, type1s and type2s were found to follow statistically the same tight MIR -- hard X-ray correlation, suggesting the MIR emission is highly isotropic assuming the hard X-ray radiation is inclination independent. We argue this discrepancy could be solved considering the hard X-ray emission in AGN is also mildly anisotropic as we recently discovered. To verify this diagram, we compare the sub-arcsecond 12μm flux densities of type1s and type2s using [OIV]$λ$25.89μm emission line as an isotropic luminosity indicator. We find that on average type1s are brighter in nuclei 12μm radiation by a factor of $2.6 \pm 0.6$ than type2s at given [OIV]$λ$25.89μm luminosities, confirming the mild anisotropy of the nuclei 12μm emission. We show that the anisotropy of the 12μm emission we detected is in good agreement with radiative transfer models of clumpy torus. The fact that type 1 and type 2 AGNs follow the same tight MIR -- hard X-ray correlation instead supports that both the MIR and hard X-ray emission in AGNs are mildly anisotropic.