A mid-IR comparative analysis of the Seyfert galaxies NGC 7213 and NGC 1386

Abstract

New Gemini mid-infrared spectroscopic observations together with Spitzer Space telescope archival data are used to study the properties of the dusty torus and circumnuclear star formation in the active galaxies NGC 7213 and NGC 1386. Our main conclusions can be summarized as follows. Polycyclic aromatic hydrocarbon (PAH) emission is absent in the Thermal-Region Camera and Spectrograph (T-ReCS) nuclear spectra but is ubiquitous in the data from Spitzer at distances above 100 pc. Star formation rates surface densities are estimated from the 12.8 μm [Ne II] line strengths leading to values close to 0.1Mʘyr-ˡ kpc-². Analogous estimates based on photometric fluxes of Infrared Array Camera’s 8 μm images are higher by a factor of almost 15, which could be linked to excitation of PAH molecules by older stellar populations. T-ReCS high-spatial-resolution data reveal silicate absorption at λ 9.7 μm in the central tens of parsecs of the Seyfert 2 NGC 1386 and silicate emission in the Seyfert 1 galaxy NGC 7213. In the case of NGC 1386, this feature is confined to the inner 20 pc, implying that the silicate might be linked to the putative dusty torus. Finally, by fitting CLUMPY models to the T-ReCS nuclear spectra, we estimate the torus physical properties for both galaxies, finding line-of-sight inclinations consistent with the AGN unified model.