Empirical diagnostics of the starburst-AGN connection

We examine a representative sample of 35 Seyfert 2 nuclei. Previous work ha s shown that nearly half (15) of these nuclei show the direct (but difficul t to detect) spectroscopic signature at optical/near-UV wavelengths of the hot massive stars that power circumnuclear starbursts. In the present paper we examine a variety of more easily measured quantities for this sample, s uch as the equivalent widths of strong absorption features, continuum color s, emission line equivalent widths, emission line ratios and profiles, far- IR luminosities, and near-UV surface brightness. We compare the composite s tarburst + Seyfert 2 nuclei to "pure" Seyfert 2 nuclei, Starburst galaxies, and normal galactic nuclei. Our goals are to verify whether the easily mea sured properties of the composite nuclei are consistent with the expected i mpact of a starburst and to investigate alternative less demanding methods to infer the presence of starbursts in Seyfert 2 nuclei, applicable to larg er or more distant samples. We show that starbursts do indeed leave clear a nd easily quantifiable imprints on the near-UV to optical continuum and emi ssion line properties of Seyfert 2s. Composite starburst + Seyfert 2 system s can be recognized by: (1) a strong "featureless continuum" (FC), which di lutes the Ca II K line from old stars in the host's bulge to an equivalent width W-K < 10 Angstrom; (2) emission lines whose equivalent widths are int ermediate between starburst galaxies and "pure" Seyfert 2's; (3) relatively low excitation line ratios, which indicate that part of the gas ionization in these Seyfert 2s (typically similar to 50% of H beta) is due to photoio nization by OB stars; (4) large far-IR luminosities (greater than or simila r to 10(10) L-circle dot); (5) high near-UV surface brightness (similar to 10(3) L-circle dot pc(-2)). These characteristics are all consistent with t he expected impact of circumnuclear starbursts on the observed properties o f Seyfert 2's. Furthermore, they offer alternative empirical diagnostics of the presence of circumnuclear starbursts from a few easily measured quanti ties.