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

TL;DR

This paper investigates the anisotropy of mid-infrared and hard X-ray emissions in active galactic nuclei, finding both are mildly anisotropic and consistent with radiative transfer models of a clumpy torus, resolving previous observational discrepancies.

Contribution

It demonstrates that both MIR and hard X-ray emissions in AGNs are mildly anisotropic, aligning with radiative transfer models, and explains the MIR brightness difference between type 1 and type 2 AGNs.

Findings

Type 1 AGNs are brighter in 12μm emission by a factor of 2.6 compared to type 2s at the same [OIV] luminosity.

MIR and hard X-ray emissions in AGNs are both mildly anisotropic.

The observed anisotropy aligns with models of a clumpy torus.

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 12\mu m flux densities of type1s and type2s using [OIV]$\lambda$25.89\mu m emission line as an isotropic luminosity indicator. We find that on average type1s are brighter in nuclei 12\mu m radiation by a factor of $2.6 \pm 0.6$ than type2s at given [OIV]$\lambda$25.89\mu m luminosities, confirming the mild anisotropy of the nuclei 12\mu m emission. We show that the anisotropy of the 12\mu m 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.