Stellar Ionization of Low-Luminosity Active Galactic Nuclei

Summary

Low-Ionization Nuclear Emission-Line Regions (LINERs) are a common constituent of galaxies, and are often regarded as a weak form of Seyfert activity. LINERs have emission-line luminosities that are similar to those of giant HII regions, however, and recent theoretical work suggests that their nebular properties can be reproduced in many cases with photoionization by normal O stars. In an extension of this scenario, energetic phenomena such as non-thermal radio emission, broad Hα features, and substantial X-ray luminosity seen in some LINERs might be attributable to supernovae. In this review I consider the empirical evidence bearing on an interpretation of LINERs as stellar-powered sources. While stellar phenomena appear capable of matching LINERs of modest luminosity in terms of broad-band energetics, some important differences remain in detailed spectral characteristics, particularly at X-ray energies. A certain amount of anomalous behavior on the part of stars within galaxy nuclei (e.g. in terms of the initial mass function) is required if LINERs result from stellar ionization.

Introduction

Spectroscopic surveys indicate that ∼ 30% or more of bright galaxies contain weak emission-line nuclei that are classified as LINERs (Low-Ionization Nuclear Emission-Line Regions; Heckman 1980b, hereafter H80b). The intensity ratios of low-ionization optical lines relative to recombination features are characteristically higher in LINERs than in HII regions, causing LINERs to be regarded as “active” nuclei subject to unusual energetic processes. The luminosities of LINERs and giant HII regions are comparable, however, with typical Hα luminosity ∼ 10⁴⁰ erg s⁻¹ or somewhat less.