The Stellar Populations of Low-Luminosity Active Galactic Nuclei. I. Ground-based Observations

We present a spectroscopic study of the stellar populations of low-luminosity active galactic nuclei (LLAGNs). Our main goal is to determine whether the stars that live in the innermost (100 pc scale) regions of these galaxies are in some way related to the emission-line properties, which would imply a link between the stellar population and the ionization mechanism. High signal-to-noise ratio, ground-based long-slit spectra in the 3500-5500 Å interval were collected for 60 galaxies: 51 LINERs and LINER/H II transition objects, two starburst galaxies, and seven nonactive galaxies. In this paper, the first of a series, we (1) describe the sample; (2) present the nuclear spectra; (3) characterize the stellar populations of LLAGNs by means of an empirical comparison with normal galaxies; (4) measure a set of spectral indices, including several absorption-line equivalent widths and colors indicative of stellar populations; and (5) correlate the stellar indices with emission-line ratios that may distinguish between possible excitation sources for the gas. Our main findings are as follows: (1) Few LLAGNs have a detectable young (≲107 yr) starburst component, indicating that very massive stars do not contribute significantly to the optical continuum. In particular, no features due to Wolf-Rayet stars were convincingly detected. (2) High-order Balmer absorption lines of H I (HOBLs), on the other hand, are detected in ~40% of LLAGNs. These features, which are strongest in 108-109 yr intermediate-age stellar populations, are accompanied by diluted metal absorption lines and bluer colors than other objects in the sample. (3) These intermediate-age populations are very common (~50%) in LLAGNs with relatively weak [O I] emission ([O ]/Hα ≤ 0.25) but rare (~10%) in LLAGNs with stronger [O I]. This is intriguing since LLAGNs with weak [O I] have been previously hypothesized to be "transition objects" in which both an AGN and young stars contribute to the emission-line excitation. Massive stars, if present, are completely outshone by intermediate-age and old stars in the optical. This happens in at least a couple of objects where independent UV spectroscopy detects young starbursts not seen in the optical. (4) Objects with predominantly old stars span the whole range of [O I]/Hα values, but (5) sources with significant young and/or intermediate-age populations are nearly all (~90%) weak-[O I] emitters. These new findings suggest a link between the stellar populations and the gas ionization mechanism. The strong-[O I] objects are most likely true LLAGNs, with stellar processes being insignificant. However, the weak-[O I] objects may comprise two populations, one where the ionization is dominated by stellar processes and another where it is governed by either an AGN or a more even mixture of stellar and AGN processes. Possible stellar sources for the ionization include weak starbursts, supernova remnants, and evolved poststarburst populations. These scenarios are examined and constrained by means of complementary observations and detailed modeling of the stellar populations in forthcoming communications.