The nuclear and circumnuclear stellar population in Seyfert 2 galaxies: Implications for the starburst-active galactic nucleus connection

We report the results of a spectroscopic investigation of a sample of 20 of the brightest type 2 Seyfert nuclei. Our goal is to search for the direct spectroscopic signature of massive stars and thereby probe the role of circumnuclear starbursts in the Seyfert phenomenon. The method used is based on the detection of the higher order Balmer lines and He I lines in absorption and the Wolf-Rayet feature at similar to 4680 Angstrom in emission. These lines are strong indicators of the presence of young (a few Myr) and intermediate-age (a few 100 Myr) stellar populations. In over half the sample, we have detected He I and/or strong stellar absorption features in the high-order (near-UV) Balmer series together with relatively weak lines from an old stellar population. In three others we detect a broad emission feature near 4680 Angstrom that is most plausibly ascribed to a population of Wolf-Rayet stars (the evolved descendants of the most massive stars). We therefore conclude that the blue and near-UV light of over half of the sample is dominated by young and/or intermediate age stars. The young Seyfert 2 galaxies have larger far-IR luminosities, cooler mid/far-IR colors, and smaller [O III]/H beta flux ratios than the old ones. These differences are consistent with a starburst playing a significant energetic role in the former class. We consider the possibility that there may be two distinct subclasses of Seyfert 2 nuclei (starbursts and hidden broadline regions II [BLRs]). However, the fact that hidden BLRs have been found in three of the young nuclei argues against this and suggests that nuclear starbursts may be a more general part of the Seyfert phenomenon.