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 circ umnuclear starbursts in the Seyfert phenomenon. The method used is based on the detection of the higher order Balmer lines and He I lines in absorptio n 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 inter mediate-age (a few 100 Myr) stellar populations. In over half the sample, w e 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 o ld stellar population. In three others we detect a broad emission feature n ear 4680 Angstrom that is most plausibly ascribed to a population of Wolf-R ayet stars (the evolved descendants of the most massive stars). We therefor e conclude that the blue and near-UV light of over half of the sample is do minated by young and/or intermediate age stars. The "young" Seyfert 2 galax ies 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 consi stent with a starburst playing a significant energetic role in the former c lass. 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.