Three LINERs Under the Hubble Spectral Microscope

The majority of low-ionization nuclear emission regions (LINERs) harbor supermassive black holes (SMBHs) with very low accretion rates. Since SMBHs spend most of their lifetimes in these low accretion-rate states, understanding LINERs is important for understanding active galactic nuclei (AGN) in the context of galaxy evolution. On scales of ~100 pc, the energy budget of LINERs appears to be deficient when the only source of power considered is the AGN. Thus, other energy sources are likely to contribute to the excitation of the emission-line gas. To probe these sources, we observed three nearby, bright, and representative LINERs with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST). We specifically looked at the 0.1-1 arcsecond scale (corresponding to ~5-50 pc) to find what these energy sources are and how far from the nucleus they take over the excitation of the gas. After subtracting both the unresolved nuclear light and the spatially-extended starlight, we measured a number of diagnostic emission line ratios. We find that within 10 pc, the observed line ratios of [O III]/[O II] vs [O IIII]/H-beta are consistent with AGN photoionization. The line ratios of [N II]/H-alpha vs [O III]/H-beta show that at larger distances, the excitation mechanism is most likely consistent with hot stars or shocks. Shocks are particularly appealing in the case of NGC 4278, which harbors a Gigahetz-Peaked radio source with small jets detected by the VLBA. If hot stars are the primary excitation mechanism, these are most likely post-AGB stars (from the old stellar population). We conclude from these representative cases that the characteristic LINER emission-line spectrum does not result from a single excitation mechanism, but rather from a combination of different mechanisms within the central 100 pc of each object that varies from object to object.