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 aboard the Hubble Space Telescope(HST), have revealed an ultraviolet (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. Firstly, 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 C IV λ1548 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 ∼500 M_⊙ 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 10⁵ yr.