CAHA/PPAK Integral-field Spectroscopic Observations of M81. I. Circumnuclear Ionized Gas

Abstract

Galactic circumnuclear environments of nearby galaxies provide unique opportunities for our understanding of the coevolution between supermassive black holes and their host galaxies. Here, we present a detailed study of ionized gas in the central kiloparsec region of M81, which hosts the closest prototype low-luminosity active galactic nucleus, based on optical integral-field spectroscopic observations taken with the CAHA 3.5 m telescope. It is found that much of the circumnuclear ionized gas is concentrated within a bright core of ~200 pc in extent and a surrounding spiral-like structure known as the nuclear spiral. The total mass of the ionized gas is estimated to be ~2 × 10⁵ M _⊙, which corresponds to a few percent of the cold gas mass in this region, as traced by co-spatial dust extinction features. Plausible signature of a biconical outflow along the disk plane is suggested by a pair of blueshifted/redshifted low-velocity features, symmetrically located at ~120-250 pc from the nucleus. The spatially resolved line ratios of [N II]/Hα and [O III]/Hβ demonstrate that much of the circumnuclear region can be classified as a LINER. However, substantial spatial variations in the line intensities and line ratios strongly suggest that different ionization/excitation mechanisms, rather than just a central dominant source of photoionization, are simultaneously at work to produce the observed line signatures.