A Compton thick AGN in the barred spiral NGC 4785

TL;DR

This study confirms NGC 4785 hosts a Compton thick AGN through X-ray observations, revealing its spectral properties, high obscuration, and complex host galaxy features, highlighting challenges in identifying such AGN via mid-infrared data.

Contribution

First detailed X-ray analysis of NGC 4785 confirming it as a Compton thick AGN with unique host galaxy characteristics.

Findings

NGC 4785 is a bona fide Compton thick AGN with Nh ≥ 2×10^{24} cm^{-2}.

The AGN exhibits a flat continuum and strong neutral iron fluorescence line.

Mid-infrared properties of NGC 4785 differ from typical Compton thick AGN.

Abstract

We present X-ray observations of the active galactic nucleus (AGN) in NGC 4785. The source is a local Seyfert 2 which has not been studied so far in much detail. It was recently detected with high significance in the 15-60 keV band in the 66 month Swift/BAT all sky survey, but there have been no prior pointed X-ray observations of this object. With Suzaku, we clearly detect the source below 10 keV, and find it to have a flat continuum and prominent neutral iron fluorescence line with equivalent width >~1 keV. Fitting the broadband spectra with physical reflection models shows the source to be a bona fide Compton thick AGN with Nh of at least 2x10^{24} cm^{-2} and absorption-corrected 2-10 keV X-ray power L(2-10) ~ few times 10^{42} erg s^{-1}. Realistic uncertainties on L(2-10) computed from the joint confidence interval on the intrinsic power law continuum photon index and normalization are at least a factor of 10. The local bona fide Compton thick AGN population is highly heterogeneous in terms of WISE mid-infrared source colours, and the nucleus of NGC 4785 appears especially sub-dominant in the mid-infrared when comparing to other Compton thick AGN. Such sources would not be easily found using mid-infrared selection alone. The extent of host galaxy extinction to the nucleus is not clear, though NGC 4785 shows a complex core with a double bar and inner disk, adding to the list of known Compton thick AGN in barred host galaxies.