The Shocking Power Sources of LINERs
  The majority of low-ionization nuclear emission-line regions (LINERs) harbor
supermassive black holes with very low accretion rates. However, the accretion
flows do not produce enough ionizing photons to power the emission lines
emitted on scales of ~100 pc, and therefore additional sources of power are
required. We present and analyze Hubble Space Telescope spectra of three nearby
luminous LINERs that are spatially resolved on scales of < 9 pc. The targets
have multiple indicators of an accreting black hole, as well as a deficient
ionizing photon budget. We measure diagnostic emission line ratios as a
function of distance from the nucleus and compare them to models for different
excitation mechanisms: shocks, photoionization by the accreting black hole, and
photoionization by young or old hot stars. We also consider the kinematics of
the line-emitting gas, as revealed by the widths and shifts of the emission
lines. We conclude that, in LINERs with low-luminosity active nuclei, shocks by
jets or other outflows are crucial in exciting the gas in and around the
nucleus, as suggested by other authors. The physical model that best describes
our targets comprises a low-luminosity, accretion-powered active nucleus that
photoionizes the gas within ~20 pc of the galaxy center, and shock excitation
of the gas at larger distances.
