Ruling out conventional photoionization models in the closest LINER M31 with CFHT/SITELLE observations

TL;DR

This study tests whether standard photoionization models can explain LINER emission in M31 and finds significant discrepancies, especially in high-energy photon predictions, suggesting the need for alternative ionization sources.

Contribution

The paper provides a detailed, spatially resolved photoionization analysis of M31's LINER emission, revealing limitations of conventional stellar and AGN models in explaining observed ionization features.

Findings

Standard stellar photoionization models cannot reproduce [OIII] emission.

Enhanced AGN activity or low-density media only partially explain observations.

Persistent challenges remain in explaining LINER emission with conventional models.

Abstract

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 kpc. 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 H{\alpha}, H\b{eta}, and [NII]. However, these models fail to match the observed [OIII] emission, producing radial profiles and [O III]/H\b{eta} 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 H{\alpha}-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.