3-4 Micron Spectroscopy of Seyfert 2 Nuclei to Quantitatively Assess the Energetic Importance of Compact Nuclear Starbursts

We report on 3-4 μm slit spectroscopy of 13 Seyfert 2 nuclei. The 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission is used to estimate the magnitudes of compact nuclear starbursts (on scales less than a few hundred parsecs) and to resolve the controversy over their energetic importance in Seyfert 2 nuclei. For three selected Seyfert 2 nuclei that have been well studied in the UV, the magnitudes of the compact nuclear starbursts estimated from the 3.3 μm PAH emission (with no extinction correction) are in satisfactory quantitative agreement with those based on the UV after extinction correction. This agreement indicates that the flux attenuation of compact nuclear starburst emission due to dust extinction is insignificant at 3-4 μm, and thus allows us to use the observed 3.3 μm PAH luminosity to estimate the magnitudes of the compact nuclear starbursts in Seyfert 2 nuclei. Based directly on our 3-4 μm slit spectra, the following two main conclusions are drawn: (1) except in one case, the observed nuclear 3-4 μm emission is dominated by active galactic nuclei (AGNs) and not by starbursts, and (2) compact nuclear starbursts are detected in 6 out of 13 Seyfert 2 nuclei, but cannot dominate the energetics of the galactic infrared dust emission in the majority of the observed Seyfert 2 galaxies. For several sources for which Infrared Space Observatory spectra taken with larger apertures and/or soft X-ray data are available, these data are combined with our 3-4 μm slit spectra, and it is suggested that (3) extended (kpc scale)...