Mid-infrared properties of local active galactic nuclei

The local active galactic nuclei (AGN) population enables us to study in detail the structures and radiation processes in the direct environment of the supermassive black holes in galaxy centers and to develop an understanding of the general AGN phenomenon. For this purpose, the first unbiased sample of 104 local AGN has now become available thanks to Swift/BAT observations in the hard X-ray regime. We performed high-angular resolution (HR) mid-infrared (MIR) imaging observations of most of these AGN with VLT/VISIR and Gemini/Michelle in order to study the dusty tori in these objects – a key component in AGN models. At the same time, we obtained similar data of a complementary sample of AGN in the low-luminosity regime. In this thesis, I present the results of both programmes and show that unresolved (< 0.4 ) MIR emission is present in the AGN of both samples. A comparison with large aperture (∼ 30 ) data shows that the host galaxy outshines low-luminosity and moderate AGN in the MIR, which emphasizes the necessity of HR to isolate and investigate the MIR properties of AGN. Hence, many Spitzer/IRS spectra are affected by non-nuclear MIR emission. Nevertheless, I can use the polycyclic aromatic hydrocarbon emission feature strength determined from these spectra to constrain the maximum star formation contribution to the HR nuclear emission to be minor. Instead, the observed unresolved MIR nuclei are consistent with thermal emission from dust in the direct vicinity of the AGN. In particular, the average MIR spectral energy distributions show the silicate feature in emission for type 1 AGN and in absorption for type 2 AGN. Furthermore, the MIR emission is strongly correlated with the absorption-corrected 2-10 keV X-ray emission over the whole range of luminosities (1040 to 1045 erg/s) which is independent of the accretion rates over a large range (Lbol /LEdd ∼ 10−3.8 to 1). On the other hand, objects with Lbol /LEdd 10−3.8 exhibit high MIR–X-ray luminosity ratios indicating contribution of an additional component. However, they still follow well the correlation of the other AGN. A similar correlation (of the MIR) with the observed 14-195 keV emission is found. Both correlations are strongest for type 1 AGN while their slope and normalization do not show any dependence on the nuclear obscuration or host properties. On the contrary, highly obscured objects occur more often in edge-on galaxies than unobscured AGN. This indicates that in some cases the obscuration of the AGN is caused by non-nuclear structures in the host galaxy. Nevertheless, the fact that MIR–X-ray ratio is very uniform for all AGN types and host galaxies indicates that the MIR emission is coming from a dusty nuclear structure that is directly heated by the accretion disk. Together with the successful explanation of the findings from previous HR MIR data of AGN with models of dusty clumpy tori, this indicates that such a structure is present in all of the AGN from the whole probed luminosity range. Therefore, the standard unification model may be valid for all types of AGN without major modifications.