The Nuclear and Circumnuclear Stellar Population in Seyfert 2 Galaxies: Implications for the Starburst–Active Galactic Nucleus Connection

Abstract

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 HeI lines in absorption and the Wolf-Rayet feature at $\sim$4680 \AA in emission. These lines are strong indicators of the presence of young (a few Myrs) and intermediate-age (a few 100 Myrs) stellar populations. In over half the sample, we have detected HeI and/or strong stellar absorption features in the high-order (near-UV) Balmer series together with relatively weak lines from an old stellar population. In three others we detect a broad emission feature near 4680 \AA that is most plausibly ascribed to a population of Wolf-Rayet stars (the evolved descendants of the most massive stars). We therefore conclude that the blue and near-UV light of over half of the sample is dominated by young and/or intermediate age stars. The ``young'' Seyfert 2's have have larger far-IR luminosities, cooler mid/far-IR colors, and smaller [OIII]/H$\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 sub-classes of Seyfert 2 nuclei (``starbursts'' and ``hidden BLR''). 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.