CAHA/PPAK Integral-field Spectroscopic Observations of M81. II. Testing Photoionization Models in a Spatially Resolved LINER
Abstract
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 Hα, Hβ, 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]/Hβ 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, Hα-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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 2023
- DOI:
- 10.3847/1538-4357/ad0299
- arXiv:
- arXiv:2310.07304
- Bibcode:
- 2023ApJ...958...89L
- Keywords:
-
- Active galactic nuclei;
- Photoionization;
- Galaxy nuclei;
- Galaxy spectroscopy;
- 16;
- 2060;
- 609;
- 2171;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 27 pages, 16 figures, accepted for publication in ApJ. Comments welcome