The 3.4-mu m absorption feature towards three obscured active galactic nuclei

The results of 3-4-mum spectroscopy towards the nuclei of NGC 3094, 7172, a nd 7479 are reported. In ground-based 8-13-mum spectra, all the sources hav e strong absorption-like features at similar to 10 mum, but they do not hav e detectable polycyclic aromatic hydrocarbon (PAH) emission features. The 3 .4-mum carbonaceous dust absorption features are detected towards all nucle i. NGC 3094 shows a detectable 3.3-mum PAH emission feature, while NGC 7172 and 7479 do not. Nuclear emission whose spectrum shows dust absorption fea tures but no PAH emission features, is thought to be dominated by highly ob scured active galactic nuclei (AGNs) activity. For NGC 7172, 7479, and thre e other such nuclei in the literature, we investigate the optical depth rat ios between the 3.4-mum carbonaceous dust and 9.7-mum silicate dust absorpt ion (tau (3.4)/tau (9.7)) The tau (3.4)/tau (9.7) ratios towards three high ly obscured AGNs with face-on host galaxies are systematically larger than the ratios in the Galactic diffuse interstellar medium or the ratios for tw o highly obscured AGNs with edge-on host galaxies. We suggest that the larg er ratios can be explained if the obscuring dust is so close to the central AGNs that a temperature gradient occurs in it. If this idea is correct, ou r results may provide spectroscopic evidence for the presence of the putati ve 'dusty tori' in the close vicinity of AGNs.