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

Abstract

In the center of active galactic nuclei (AGNs), the dusty torus absorbs the radiation from the central engine and reemits 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 that the MIR emission is highly isotropic assuming that the hard X-ray radiation is inclination independent. We argue that this discrepancy could be solved considering that the hard X-ray emission in AGNs is also mildly anisotropic, as we recently discovered. To verify this diagram, we compare the subarcsecond 12 μm flux densities of type1s and type2s using the [O iv] λ25.89 μm emission line as an isotropic luminosity indicator. We find that on average type1s are brighter in nuclei 12 μm radiation by a factor of 2.6 ± 0.6 than type2s at given [O iv] λ25.89 μm luminosities, confirming the mild anisotropy of the nuclei 12 μm emission. We show that the anisotropy of the 12 μm emission we detected is in good agreement with radiative transfer models of clumpy tori. The fact that type1s and type2s follow the same tight MIR-hard X-ray correlation instead supports that both the MIR emission and hard X-ray emission in AGNs are mildly anisotropic.

Luminosity Ratio between [O iv] 25.89 μm Line and Nuclear Continuum 12 μm as a Diagnostic for “Buried” AGNs

“…The merger stages are distinguished with colors. We find that the non-mergers and pre-/earlystage mergers, most of which are Seyfert 1.8/2s or AGNimportant composites (Table 2), follow the same correlation as for typical Seyfert 2s (the black solid line; Yang et al 2015). By contrast, the mid-and late-stage mergers, most of which are composites, show smaller…”

“…Figure 1(a) plots the luminosity correlations between the [O IV] 25.89 µm line and nuclear 12 µm continuum for the AGNs in Table 1. As noticed, the low X-ray-scattering AGNs (f scat < 0.5%) show smaller [O IV] luminosities relative to the nuclear 12 µm ones than the average of Seyfert 2s obtained by Yang et al (2015). Figure 1…”

“…We find that the L [O IV] /L bution between the low X-ray-scattering AGNs and the others is significant at a >99% confidence level. Yang et al (2015) suggest that both X-ray and continuum MIR emission in AGNs may be mildly anisotropic by assuming that the [O IV] 25.89 µm line is an isotropic luminosity indicator; AGNs viewed at higher inclination angles tend to have smaller X-ray and MIR luminosities. This would lead to an apparent correlation between f scat (∝ 1/L X ) and L [O IV] /L (nuc) 12 µm .…”

“…Arrows mark upper limits in the case of MIR non-detection. Black-solid and blue-dashed lines show the averaged relation for Seyfert 2s obtained by Yang et al (2015) and that corrected for contribution of starburst in the MIR luminosity by a factor of 3. (b) The ratio of [O IV] to nuclear 12 µm luminosities vs. merger stage.…”

Obscuration in active galactic nuclei: near-infrared luminosity relations and dust colors

“…9, bottom center) that would also indicate that the mid-IR emission is anisotropic (as also seen by Yang et al 2015). However, this offset is not significant.…”