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

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 Hα, 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 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.

Figures (10)

• Figure 1: $Left$: H$\alpha$ emission from SITELLE observations, overlaid with [OIII] contours with levels 0.3, 1.0, 3.0, 5.0$\times 10^{-16}~\rm erg~s^{-1}~cm^{-2}~arcsec^{-2}$, where the lowest contour corresponds to a $5\sigma$ detection.$Right:$ Example spectrum extracted from the red circle shown in the left panel. This region lies at the edge of the nuclear spiral and is representative of low surface brightness emission. The original SN2 and SN3 spectra are shown in black, and the stellar continuum model fitted with pPXF is shown in red. After continuum subtraction, the major emission lines become clearly visible in the residual spectrum shown in orange.
• Figure 2: $Left~and~middle:$ Conventional BPT diagrams of the bulge region. The dividing lines are taken from 2006MNRAS.372..961K, which separate the regions into Seyfert, star formation (SF), LINER, and composite categories, corresponding to the main ionization mechanisms. The [OI]/H$\alpha$ diagram is not shown because the observations do not cover the [OI]$\lambda 6300$ line. Each data point represents a Voronoi bin, and the errorbar represents the statistical uncertainty of the data. The points are color-coded with the deprojected radius from the center. $Right:$ The H$\alpha$ equivalent width versus [NII]/H$\alpha$ (WHAN) diagram 2011MNRAS.413.1687C of the bulge region. This diagram further separates the traditional LINER region into weak AGN (LINER) and passive galaxies (ionized by HOLMES).
• Figure 3: A cartoon of the adopted geometry. Radially spaced clouds are distributed in the plane of the nuclear gas disk (nuclear spiral in M31's case) and are illuminated by the combined radiation field of bulge and disk stars.
• Figure 4: Spectral energy distribution (SED) of M31* (dotted blue curve) and stellar populations with age 10 Gyr and metallicity 1.5 $Z_\odot$ (solid black curve). Additional SP models are shown for (age = 1 Gyr, $Z = 1.5\,Z_\odot$), (age = 10 Gyr, $Z = 2\,Z_\odot$), and a composite population consisting of 90% 10 Gyr + 10% 1 Gyr at $Z = 1.5\,Z_\odot$, plotted with green dashed, orange dotted, and red dash-dotted curves, respectively.
• Figure 5: The radial distribution of line intensities of the four major emission lines H$\alpha$, H$\beta$, [OIII], and [NII] compared with CLOUDY models B1-2. Data extracted from each Voronoi bin is shown in black points, while the CLOUDY model A with a constant density representative of the nuclear gas disk as described above, is shown with a red solid line. Model B1 and B2 with lower density of $n= 50~\rm cm^{-3}$ and $n=1~\rm cm^{-3}$ are shown with cyan dotted and dashed lines, respectively.