3-4 micron spectroscopy of Seyfert 2 nuclei to quantitatively assess the energetic importance of compact nuclear starbursts

We report on 3-4 mum slit spectroscopy of 13 Seyfert 2 nuclei. The 3.3 mum 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 mum 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 mum, and thus allows us to use the observed 3.3 mum PAH luminosity to estimate the magnitudes of the compact nuclear starbursts in Seyfert 2 nuclei. Based directly on our 3-4 mum slit spectra, the following two main conclusions are drawn: (1) except in one case, the observed nuclear 3-4 mum 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 mum 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, although the AGN is still an important contributor to the luminosities, 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, a correlation between the luminosities of AGNs and compact nuclear starbursts is implied; more powerful AGNs tend to be related to more powerful compact nuclear starbursts.