CAHA/PPAK Integral-field Spectroscopic Observations of M81 -- I.
  Circumnuclear ionized gas
  Galactic circumnuclear environments of nearby galaxies provide unique
opportunities for our understanding of the co-evolution between super-massive
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 3.5m telescope.
It is found that much of the circumnuclear ionized gas is concentraed within a
bright core of $\sim$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 $\sim2\times10^5\rm~M_\odot$, 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 bi-conical outflow along the disk plane is suggested
by a pair of blueshifted/redshifted low-velocity features, symmetrically
located at $\sim$ 120 -- 250 pc from the nucleus. The spatially-resolved line
ratios of [N\,{\sc ii}]/H$\alpha$ and [O\,{\sc iii}]/H$\beta$ demonstrate that
much of the circumnuclear region can be classified as LINER (low-ionization
nuclear emission-line region). 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.
