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 Å) 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]/Hα ≤ 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