Polycyclic aromatic hydrocarbon in the central region of the Seyfert 2 galaxy NGC 1808

Abstract

We present mid-infrared spectra of the Seyfert 2 galaxy NGC 1808, obtained with the Gemini’s Thermal-Region Camera Spectrograph at a spatial resolution of ∼26 pc. The high spatial resolution allowed us to detect bright polycyclic aromatic hydrocarbons (PAHs) emissions at 8.6 and 11.3 μm in the galaxy centre (∼26 pc) up to a radius of 70 pc from the nucleus. The spectra also present [Ne II]12.8 μm ionic lines, and H2 S(2) 12.27 μm molecular gas line. We found that the PAHs profiles are similar to Peeters’s A class, with the line peak shifted towards the blue. The differences in the PAH line profiles also suggest that the molecules in the region located 26 pc north-east of the nucleus are more in the neutral than in the ionized state, while at 26 pc south-west of the nucleus, the molecules are mainly in ionized state. After removal of the underlying galaxy contribution, the nuclear spectrum can be represented by a Nenkova’s CLUMPY torus model, indicating that the nucleus of NGC 1808 hosts a dusty toroidal structure with an angular cloud distribution of σ = 70◦, observer’s view angle i = 90◦ and an outer radius of R0 ∼ 0.55 pc. The derived column density along the line of sight is NH = 1.5 × 1024 cm2, which is sufficient to block the hard radiation from the active nucleus, and would explain the presence of PAH molecules near to the NGC 1808’s active nucleus.