An Assessment of the Energy Budgets of Low-Ionization Nuclear Emission
  Regions
  Using the SEDs of the weak AGNs 35 LINERs presented in a companion paper, we
assess whether photoionization by the weak AGN can power the emission-line
luminosities measured through the large (few-arcsecond) apertures used in
ground-based spectroscopic surveys. Spectra taken through such apertures are
used to define LINERs as a class and constrain non-stellar photoionization
models for LINERs. Therefore, our energy budget test is a self-consistency
check of the idea that the observed emission lines are powered by an AGN. We
determine the ionizing luminosities and photon rates by integrating the
observed SEDs and by scaling a template SED. Even if all ionizing photons are
absorbed by the line-emitting gas, more than half of our LINERs suffer from a
deficit of ionizing photons. In 1/3 of LINERs the deficit is severe. If only
10% of the ionizing photons are absorbed by the gas, there is an ionizing
photon deficit in 85% of LINERs. We disfavor the possibility that additional
electromagnetic power, either obscured or emitted in the unobservable far-UV
band, is available from the AGN. We consider other power sources such as
mechanical heating by compact jets and photoionization by young or old stars.
Photoionization by young stars may be important in a small fraction of cases.
Mechanical heating provides enough power in most cases but it is not clear how
this power is transferred to the emission-line gas. Photoionization by post-AGB
stars is an important power source; it provides more ionizing photons that the
AGN in more than half of the LINERs and enough ionizing photons to power the
emission lines in 1/3 of the LINERs. It appears likely that the emission-line
spectra of LINERs obtained from the ground include the sum of emission from
different regions where different power sources dominate.
