1995A&A...301...55O

We present infrared spectral observations of CO (1.62, 2.29µm) and Si (1.59µm) stellar absorption features in 23 galaxies including 14 calibrators (normal ellipticals/spirals and well known HII galaxies) and 9 Seyferts. Our main aim was to define and calibrate an observational method for tracing red supergiants (i.e. starbursts) in active galaxies. Additional spectra were also obtained around the Brγ hydrogen recombination line. The measured equivalent widths of the absorption features in old stellar systems and HII galaxies are found to be remarkably similar and the CO and Si indices do not therefore provide reliable diagnostics for distinguishing red supergiants (i.e. starbursts) from metallic red giants (i.e. old stellar systems). A more sensitive quantity is found to be the light to mass ratio (L_H/M) as inferred from the observed 1.65µm stellar luminosities and velocity dispersions. All HII galaxies and several of the Seyfert 2's exhibit L_H/M values a factor ≃5 larger than normal ellipticals and spirals. Both the L_H/M ratios and the Brγ equivalent widths in Sy2's are consistent with the presence of starbursts which are older than those in HII galaxies. A large fraction of the near infrared (<2µm) continuum in Sy1's also appears to be stellar but associated with even older starburst activity or normal red giants in the galaxy bulge. These results therefore provide additional support for evolutionary models in which Seyfert activity is related to the presence of a black hole which is formed/fueled by the remnants of a pre-cursor starburst. The origin of the non-stellar 2µm continuum in Seyferts and the metallicity of ellipticals are also discussed.