Dissecting the Power Sources of Low-Luminosity Emission-Line Galaxy
  Nuclei via Comparison of HST-STIS and Ground-Based Spectra
  Using a sample of ~100 nearby line-emitting galaxy nuclei, we have built the
currently definitive atlas of spectroscopic measurements of H_alpha and
neighboring emission lines at subarcsecond scales. We employ these data in a
quantitative comparison of the nebular emission in Hubble Space Telescope (HST)
and ground-based apertures, which offer an order-of-magnitude difference in
contrast, and provide new statistical constraints on the degree to which
Transition Objects and low-ionization nuclear emission-line regions (LINERs)
are powered by an accreting black hole at <10 pc. We show that while the
small-aperture observations clearly resolve the nebular emission, the aperture
dependence in the line ratios is generally weak, and this can be explained by
gradients in the density of the line-emitting gas: the higher densities in the
more nuclear regions potentially flatten the excitation gradients, suppressing
the forbidden emission. The Transition Objects show a threefold increase in the
incidence of broad H_alpha emission in the high-resolution data, as well as the
strongest density gradients, supporting the composite model for these systems
as accreting sources surrounded by star-forming activity. The narrow-line
LINERs appear to be the weaker counterparts of the Type 1 LINERs, where the low
accretion rates cause the disappearance of the broad-line component. The
enhanced sensitivity of the HST observations reveals a 30% increase in the
incidence of accretion-powered systems at z~0. A comparison of the strength of
the broad-line emission detected at different epochs implies potential
broad-line variability on a decade-long timescale, with at least a factor of
three in amplitude.
