Compact nuclear starbursts in Seyfert 2 galaxies from the CfA and 12 micron samples

We present infrared 2.8-4.1 mum slit spectra of 32 Seyfert 2 galaxies in the CfA and 12 mum samples. The 3.3 mum polycyclic aromatic hydrocarbon (PAH) emission feature was used to estimate the absolute magnitude of a compact nuclear starburst ( less than a few hundred parsecs in size) that is presumed to have occurred in the outer region of an obscuring dusty molecular torus around a central supermassive black hole. We detected 3.3 mum PAH emission in 11 of the 32 Seyfert 2 nuclei in our sample, providing evidence for the presence of compact nuclear starbursts in a significant fraction of Seyfert 2 nuclei. However, the rest-frame equivalent widths of the 3.3 mum PAH emission and the 3.3 mum PAH-to-infrared luminosity ratios measured in this study suggest that compact nuclear starbursts generally do not contribute significantly to the observed 3-4 mum nuclear fluxes or to the infrared luminosities of Seyfert 2 galaxies. Absorption features at 3.4 mum from bare dust were clearly detected in only two of the nuclei, and features at 3.1 mum from ice-covered dust were detected in only one nucleus. If the dust properties in the direction of these Seyfert 2 nuclei do not differ significantly from the Galactic interstellar medium, then these small absorption optical depths suggest that dust extinction toward the 3-4 mum continuum emitting region in the innermost part of the obscuring dusty torus is modest: A(V) < 50-60 mag. Finally, the 3.3 mu m PAH emission luminosities measured in this study were found to be significantly correlated with IRAS 12 and 25 mu m and nuclear N-band (10.6 mu m) luminosities. If these three luminosities trace the power of the active galactic nucleus (AGN), then the luminosities of compact nuclear starbursts and AGNs are correlated. This correlation is in agreement with theories predicting that the presence of a compact nuclear starburst in the torus leads to an enhancement of the mass accretion rate onto the central supermassive black hole.