3--4 $\mu$m Spectroscopy of Seyfert 2 Nuclei to Quantitatively Assess the Energetic Importance of Compact Nuclear Starbursts

We report on 3–4 $\mu$m slit spectroscopy of 13 Seyfert 2 nuclei. The 3.3 $\mu$m polycyclic aromatic hydrocarbon (PAH) emission is used to estimate the magnitudes of compact ($<$ a few 100 pc) nuclear starbursts 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 $\mu$m PAH emission (with no extinction correction) are in satisfactory quantitative agreement with those based on the UV after extinction correction. Based directly on our 3–4 $\mu$m slit spectra, the following two main conclusions are drawn: (1) except in one case, the observed nuclear 3–4 $\mu$m emission is dominated by AGN 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 $\mu$m slit spectra, and it is suggested that (3) extended (kpc scale) star-formation activity is energetically more important than compact nuclear starbursts, and contributes significantly to the infrared luminosities of Seyfert 2 galaxies, and (4) the bulk of the energetically significant extended star-formation activity is of starburst type rather than quiescent normal disk star-formation; the extended starbursts are responsible for the superwind-driven soft X-ray emission from Seyfert 2 galaxies. Finally, it is implied that more powerful AGNs tend to be related to more powerful compact nuclear starbursts.