LINERs Under the Microscope: Tracing Mechanical Feedback from Low-Luminosity AGN

Low-ionization nuclear emission regions (LINERs) are observed in more than half of nearby galaxies, and the majority of nearby LINERs harbor low-luminosity active galactic nuclei (LLAGN). Despite the ubiquity of LINERs, the mechanisms for gas excitation and emission within them are poorly understood – prior studies find that, on large scales, photoionization from an LLAGN alone is unable to account for observed emission but, on small scales, an LLAGN is sufficiently luminous to photoionize the gas. Beyond ~10 parsecs, shock excitation must power the emission. Thus, to test the universality of the shock interpretation and the applicability of other power sources (e.g. supernovae shocks), and to investigate how LLAGN impart energy onto their hosts, we expand on prior small studies and present Hubble Space Telescope STIS spectra of a diverse set of 15 LINERs. We map emission line diagnostics and the kinematic behavior of LINER gas as a function of galactocentric distance, and find evidence for a change in emission mechanism with increasing distance. If observed shock emission is indeed caused by outflows, then LINERs may represent a population where mechanical feedback from an LLAGN exerts significant influence on the interstellar medium of the host. In this context, we place lower limits on the mechanical power required to excite LINER emission and find that, in some cases, the minimum mechanical power is on the same order as the bolometric luminosity of the LLAGN.