Inflowing Gas in the Central Parsec of M81
  Spectroscopic observations of the Seyfert 1/Liner nucleus of M81, obtained
recently with the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble
Space Telescope (HST), have revealed a UV--visible spectrum rich with emission
lines of a variety of widths, ionization potentials, and critical densities,
including several in the UV that have not previously been reported. Even at the
highest angular resolution currently achievable with HST, the broad-line region
of M81 cannot be uniquely defined on the basis of commonly used observables
such as the full-width at half maximum of the emission lines, or ratios of
various emission lines. Numerous broad forbidden lines complicate
interpretation of the spectra. At least three separate line-emitting components
are inferred. A large, highly ionized, low density, low metallicity H${^+}$
region producing the broad Balmer lines. Located within the H${^+}$ region are
smaller condensations spanning a wide-range in density, and the source of
forbidden line emission through collisional excitation of the respective ions.
Intermingled with the H${^+}$ region and the condensations is a curious
extended source of time-variable CIV ${\lambda}$ 1548 emission. Collectively,
these observations can be qualitatively understood in the context of a shock
excited jet cavity within a large H${^+}$ region that is photoionized by the
central UV--X-ray source. The H${^+}$ region contains ${\sim}$ 500 M${\odot}$
of low metallicity gas that is dynamically unstable to inflow. At the current
rate, the available H${^+}$ gas can sustain the advection dominated accretion
flow that powers the central UV--X-ray source for 10$^{5}$ years.
