The stellar populations of low-luminosity active galactic nuclei.: II.: Space telescope imaging spectrograph observations

We present a study of the stellar populations of low-luminosity active galactic nuclei (LLAGNs). Our goal is to search for spectroscopic signatures of young and intermediate-age stars and to investigate their relationship with the ionization mechanism in LLAGNs. The method used is based on the stellar population synthesis of the optical continuum of the innermost ( 20 - 100 pc) regions in these galaxies. For this purpose, we have collected high spatial resolution optical (2900 - 5700 Angstrom) STIS spectra of 28 nearby LLAGNs that are available in the Hubble Space Telescope archive. The analysis of these data is compared with a similar analysis also presented here for 51 ground-based spectra of LLAGNs. Our main findings are as follows: ( 1) No features due to Wolf-Rayet stars were convincingly detected in the STIS spectra. ( 2) Young stars contribute very little to the optical continuum in the ground-based aperture. However, the fraction of light provided by these stars is higher than 10% in most of the weak-[O I] ([O I]/Halpha less than or equal to 0.25) LLAGN STIS spectra. (3) Intermediate-age stars contribute significantly to the optical continuum of these nuclei. This population is more frequent in objects with weak than with strong [O I]. Weak -[O I] LLAGNs that have young stars stand out for their intermediate-age population. (4) Most of the strong -[O I] LLAGNs have predominantly old stellar population. A few of these objects also show a featureless continuum that contributes significantly to the optical continuum. These results suggest that young and intermediate-age stars do not play a significant role in the ionization of LLAGNs with strong [ O I]. However, the ionization in weak -[O I] LLAGNs with young and/or intermediate-age populations could be due to stellar processes. A comparison of the properties of these objects with Seyfert 2 galaxies that harbor a nuclear starburst suggests that weak -[O I] LLAGNs are the lower luminosity counterparts of the Seyfert 2 composite nuclei.