Empirical Diagnostics of the Starburst‐AGN Connection

We examine a representative sample of 35 Seyfert 2 nuclei. Previous work hasshown that nearly 1/2 of these nuclei show the direct (but difficult-to-detect)spectroscopic signatures at optical-UV wavelengths of the hot massive starsthat power circum-nuclear starbursts. In this paper we examine a variety ofmore-easily-measured quantities for this sample, such as the equivalent widthsof strong absorption features, continuum colors, emission line equivalentwidths, ratios and profiles, far-IR luminosities and near-UV surfacebrightness. We compare the composite starburst+Seyfert 2 nuclei to ``pure''Seyfert 2's, Starburst galaxies and normal galactic nuclei. Our goals are toverify whether these properties in composite nuclei are consistent with theexpected impact of a starburst, and to investigate alternative less-demandingmethods to infer the presence of starbursts in Seyfert 2's, applicable tolarger or more distant samples. We show that starbursts do indeed leave clearand easily quantifiable imprints on the near-UV to optical continuum range.Composite starburst+Seyfert 2 systems can be recognized by: (1) a strong``Featureless Continuum'' which dilutes the CaII K line from the host's bulgeto W_K < 10 A; (2) emission lines whose equivalent widths are intermediatebetween Starburst galaxies and ``pure'' Seyfert 2's; (3) relatively lowexcitation line-ratios, which indicate that part of the gas ionization in theseSeyfert 2's (typically \sim 50% of Hbeta) is due to photoionization by OBstars; (4) large far IR luminosities (> 10^10 Lsun); (5) High near-UV surfacebrightness (~10^3 Lsun/pc^2). These characteristics are all consistent with theexpected impact of circum-nuclear starbursts on the observed properties ofSeyfert 2's. (abridged)Comment: ApJ in press - 67(!) pages (including 15 figures