2018ApJ...864...90M

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.