Poststarburst Models of LINERS

Since the discovery of low-ionization nuclear emission-line regions (LINERs) in many galaxies, it has been recognized that they constitute a class of active galactic nuclei (AGNs) that are thought to be powered by gas accretion onto a central, supermassive black hole. LINERs are observed in approximately one-third of galaxies in the local universe, and it has been often thought that they harbor an AGN-like central engine with moderate activity. However, some LINERs show no direct evidence for AGNs, such as broad emission lines, radio jets, hard X-ray emission, spectral energy distributions that are inconsistent with those of starlight, and so on. For such LINERs (a subset of type 2 LINERs), we present new poststarburst models that explain some of their most important optical narrow emission line ratios. In these models, the ionization sources are planetary nebula nuclei (PNNs) with temperature of ∼105 K that appear in the late-phase evolution of intermediate-mass stars with mass between ≈3 and ≈6 M⊙. Such PNNs left in a typical starburst nucleus can produce an Hα luminosity of L(Hα) ∼ 1038 ergs s-1 for typical poststarburst LINERs and ∼1039 ergs s-1 only in exceptionally bright cases. The PNN phase lasts until the death of the lowest-mass stars formed in the starburst, which is ∼5 × 108 yr for an assumed lower limit of the initial mass function of 3 M⊙. This long duration appears consistent with the observed higher frequency of occurrence of LINERs if every galaxy could experience the starburst activity several times in its life. We therefore propose that some LINERs that show no direct evidence for AGNs may be poststarburst nuclei powered by a cluster of PNNs.