  The origin of the low-ionization nuclear emission-line region (LINER)
prevalent in local galaxies and its relationship with supermassive black holes
are debated for decades. We preform 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 Halpha, Hbeta 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 [Oiii] line or an accordingly flat profile of the [O iii]/Hbeta 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, Halpha-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.
