CAHA/PPAK Integral-field Spectroscopic Observations of M81. II. Testing Photoionization Models in a Spatially Resolved LINER

The origin of the low-ionization nuclear emission-line region (LINER) prevalent in local galaxies and its relationship with supermassive black holes have been debated for decades. We perform a comprehensive evaluation of traditional photoionization models against the circumnuclear ionized gas in M81, for which recent CAHA/PPAK integral-field spectroscopic observations reveal a LINER characteristic out to a galactocentric radius of ∼1 kpc. Constructed with the photoionization code cloudy , the models have the novel aspect of their primary parameters being well constrained by extensive observations of a prototypical low-luminosity active galactic nucleus (LLAGN) and an old stellar bulge in M81. Additionally, these models incorporate a reasonably broad range of uncertain nebular properties. It is found that the integrated photoionization by the LLAGN and hot, low-mass stars distributed in the bulge can roughly reproduce the observed radial intensity distributions of the H α , H β, and [N ii ] lines, with the bulge stars dominating the ionizing flux at radii ≳200 pc. However, the models generally fail to reproduce a similarly declining profile of the [O iii ] line or an accordingly flat profile of the [O iii ]/H β ratio. This clearly points to a deficiency of ionizing photons in the outer regions despite an extended photoionization source. The discrepancy might be alleviated if much of the observed [O iii ] line arose from a bulge-filling, low-density gas surrounding a denser, H α -emitting disk, or by a higher AGN luminosity in the recent past. The case of M81 has important implications for the ionization mechanism of LINERs and low-ionization emission-line regions in general.