The 3.4 micron absorption feature towards three obscured active galactic nuclei

Abstract

The results of 3-4 $\mu$m spectroscopy towards the nuclei of NGC 3094, NGC 7172, and NGC 7479 are reported. In ground-based 8-13 $\mu$m spectra, all the sources have strong absorption-like features at $\sim$10 $\mu$m, but they do not have detectable polycyclic aromatic hydrocarbon (PAH) emission features. The 3.4 $\mu$m carbonaceous dust absorption features are detected towards all nuclei. NGC 3094 shows a detectable 3.3 $\mu$m PAH emission feature, while NGC 7172 and NGC 7479 do not. Nuclear emission whose spectrum shows dust absorption features but no PAH emission features is thought to dominated by highly obscured active galactic nuclei (AGNs) activity. For NGC 7172, NGC 7479, and three other such nuclei in the literature, we investigate the optical depth ratios between the 3.4 $\mu$m carbonaceous dust and 9.7 $\mu$m silicate dust absorption ($\tau_{3.4}$/$\tau_{9.7}$). The $\tau_{3.4}$/$\tau_{9.7}$ ratios towards three highly obscured AGNs with face-on host galaxies are systematically larger than the ratios in the Galactic diffuse interstellar medium or the ratios for two highly obscured AGNs with edge-on host galaxies. We suggest that the larger 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, our results may provide spectroscopic evidence for the presence of the putative ``dusty tori'' in the close vicinity of AGNs.