2026ApJ...999..213L

The ionization mechanisms of low-ionization nuclear emission-line regions (LINERs), which are common in the local Universe, have been debated for decades. Our nearest large neighbor, M31, is classified as a LINER based on its optical emission-line properties within the central kiloparsec. In this work, we present a detailed photoionization modeling of the circumnuclear ionized gas in M31, explicitly tailored to its well-constrained physical conditions, including the absence of ongoing star formation and a currently inactive active galactic nucleus (AGN). Using spatially resolved CFHT/SITELLE observations, we find that photoionization by hot, evolved low-mass stars distributed throughout the bulge can roughly reproduce the observed radial intensity profiles of Hα, Hβ, and [N II]. However, these models fail to match the observed [O III] emission, producing radial profiles and [O III]/Hβ ratios that are significantly steeper than observed. This discrepancy indicates a deficit of high-energy ionizing photons in standard stellar photoionization models, even with extended ionizing sources. We explore whether this tension can be alleviated by invoking either a bulge-filling, low-density ionized medium surrounding a denser Hα-emitting disk or enhanced AGN activity in the recent past. While both scenarios can partially increase the [O III] emission, neither provides a fully satisfactory explanation under physically plausible conditions. Together with our earlier results for M81, these findings underscore persistent challenges in explaining LINER-like emission solely through conventional photoionization mechanisms.