Two Physically Distinct Populations of Low-Ionization Nuclear
  Emission-Line Regions
  The nature of Low-ionization Nuclear Emission-line Regions (LINERs) has been
an open question for a long time. We study the properties of LINERs from
several different aspects. The LINERs are found to consist of two different
categories that can be clearly separated in the traditional BPT diagrams,
especially in the [OI]/Ha vs. [OIII]/Hb diagram. LINERs with high [O]/Ha ratios
(population I) differ from ones with low ratios (population II) in several
properties. Broad emission lines are only identified in the spectra of
population I LINERs. While only the population II LINERs show luminous infrared
emission and occurrence of core-collapse supernovae in the host. Combining
these results with the known distribution of stellar populations not only
suggests that the two populations have different line excitation mechanisms,
but also implies that they are at different evolutionary stages.
