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 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.
https://ui.adsabs.harvard.edu/abs/2022ApJ...928..111L/abstract https://ui.adsabs.harvard.edu/abs/2023ApJ...958...89L/abstract
BIO
Postdoctoral fellow, NAOC, 2022-present, Science contact: Di Li.
Ph.D., Astronomy, Nanjing University, 2022, advisor: Zhiyuan Li.
B.Sc., Astronomy, Nanjing University, 2017.
