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 accreti on onto a central, supermassive black hole. LINERs are observed in approxim ately one-third of galaxies in the local universe, and it has been often th ought that they harbor an AGN-like central engine with moderate activity. H owever, some LINERs show no direct evidence for AGNs, such as broad emissio n lines, radio jets, hard X-ray emission, spectral energy distributions tha t are inconsistent with those of starlight, and so on. For such LINERs (a s ubset of type 2 LINERs), we present new poststarburst models that explain s ome of their most important optical narrow emission line ratios. In these m odels, the ionization sources are planetary nebula nuclei (PNNs) with tempe rature of similar to 10(5) K that appear in the late-phase evolution of int ermediate-mass stars with mass between approximate to 3 and approximate to 6 M-circle dot. Such PNNs left in a typical starburst nucleus can produce a n H alpha luminosity of L(H alpha) similar to 10(38) ergs s(-1) for typical poststarburst LINERs and similar to 10(39) ergs s(-1) only in exceptionall y bright cases. The PNN phase lasts until the death of the lowest-mass star s formed in the starburst, which is similar to 5 x 10(8) yr for an assumed lower limit of the initial mass function of 3 M-circle dot. This long durat ion appears consistent with the observed higher frequency of occurrence of LINERs if every galaxy could experience the starburst activity several time s in its life. We therefore propose that some LINERs that show no direct ev idence for AGNs may be poststarburst nuclei powered by a cluster of PNNs.