The mid-infrared emission of Seyfert galaxies. A new analysis of ISOCAM data

TL;DR

This study analyzes mid-infrared images of 57 AGNs, revealing differences between Seyfert types, correlations with radio and X-ray emissions, and supporting the clumpy torus model of active nuclei.

Contribution

It provides a detailed morphological analysis of Seyfert galaxies in the mid-infrared, highlighting anisotropic emission and nuclear contributions distinct between types.

Findings

Nuclear emission is larger in Seyfert 1 than Seyfert 2.

Radio/infrared correlation is maintained in the sample.

Large scatter in X-ray to mid-infrared ratios supports a clumpy torus model.

Abstract

We present mid-infrared data of a sample of 57 AGNs obtained with the instrument ISOCAM on board the satellite ISO. The images were obtained through the LW2 (6.75 microns) and LW7 (9.62 microns) filters. This is a new analysis of Clavel et al. (2000) galaxy sample, which is divided into 26 type 1 (< or = 1.5) and 28 type 2 (> 1.5) Seyfert galaxies, plus three QSOs. The spatial resolution of the images allow us to separate the nuclear and the extended contributions to the total emission after decomposing the brightness profiles into different morphological components. The most common components are a central point source (identified as the active nucleus) and an exponential disk. In some cases a bulge, a bar or a ring are needed. The relative contribution of the nucleus to the total emission appears larger in Seyfert 1 than in Seyfert 2. This result confirms that both types of Seyfert galaxies are different in the mid-infrared and supports the existence of an structure which produces anisotropic emission in this wavelength range. We have also explored correlations between the mid-infrared and the radio and X-ray wavelength ranges. The well established radio/infrared correlation is mantained in our sample for the global emission of the galaxies. If only the nuclear infrared emission is considered then a non-linear correlation is apparent in the luminosity-luminosity scatter diagram. The ratio between the intrinsic hard X-ray and the nuclear mid-infrared emission presents large scatter and slightly larger values for type 2 Seyfert galaxies. These results seem to be consistent with the presence of a clumpy dusty torus surrounding the active nucleus.