Compton-thick AGN Characterisation in a Multi-wavelength Context: Insights from the 70-Month \textit{SWIFT}/BAT Catalogue

TL;DR

This study characterizes Compton-thick AGNs across multiple wavelengths, revealing their distinct properties and demonstrating that high obscuration and accretion rates drive their nature, not just orientation effects.

Contribution

It provides a comprehensive multi-wavelength analysis of CT AGNs, highlighting limitations of IR selection and proposing machine learning methods for candidate verification.

Findings

CT AGNs have higher radio luminosities, indicating active cores.

Mid-IR colours in CT AGNs are redder, but selection methods have limitations.

High Eddington ratios support radiation-driven unification of CT AGNs.

Abstract

We analyse Compton-thick active galactic nuclei (CT AGNs), a heavily obscured subclass that challenges traditional X-ray diagnostics. Using 243 sources from the 70-Month \textit{SWIFT}/BAT catalogue (26 CT, 217 non-CT), we investigate their properties across radio, infrared (IR), optical, and X-ray bands. VLASS data reveals slightly higher 2--3~GHz mean luminosities in CT AGNs, suggesting active cores attenuated by circumnuclear absorption. Mid-IR diagnostics show redder $W3-W4$ colours in CT AGNs, tracing cooler dust, with significant scatter likely driven by host-galaxy dilution. Most CT AGNs fall outside standard WISE selection wedges, highlighting mid-IR selection limitations. BPT diagnostics show that CT AGNs primarily occupy Seyfert regions, indicating isotropic narrow-line properties. CT AGNs favour significantly higher Eddington ratios ($\lambda_{\text{Edd}}$), supporting radiation-driven unification where intense accretion maintains high-column density. We also observe a moderate anti-correlation between [NII]/H$\alpha$ and $\lambda_{\text{Edd}}$. Principal component analysis identifies ionizing power and the accretion-obscuration link as primary variance drivers, though both populations overlap significantly in the PC1--PC2 plane. Machine learning achieved high recall (0.80) using intrinsic X-ray luminosity, [OI]$\lambda$6300 and H$\alpha$ luminosities, and $W2-W3$ colour. This demonstrates the potential for multi-wavelength signatures to verify CT candidates in future deep surveys where X-ray data is limited. Overall, our findings suggest CT AGNs are driven by high obscuration and accretion rates rather than a simple orientation effect.