Atomic and molecular gas in the circumnuclear disk

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Abstract
The Sgr A West HII‐region is surrounded by an extended (R≤8–9pc) massive (Md≊2–5 104M0) disk of atomic and molecular gas. The structure’s overall inclination to the line‐of‐sight is ≊70°, and the major axis is tilted against the galactic plane. The thickness of the disk seems supported by turbulent pressure. The dominant large‐scale motion is rotation (vrot≊110 kms− 1) about the nucleus. Recent high spatial resolution data reveal that the circumnuclear disk is not a planar equilibrium configuration, but is warped and kinematically perturbed on short timescales (≊105 yr− 1). The atomic and molecular line emission arises from an unusually warm (T≊300 K), dense (n≊105 cm− 3) and highly turbulent medium that occupies only a small volume of the emission region (fv≤0.1). The excitation of the gas is due to a combination of UV heating, for the atomic phase, and dissipation of turbulent energy in small‐scale low velocity shocks for the bulk of the molecular material. The disk‐like gas distribution may represent a parsec sized circumnuclear accretion disk, which in order to maintain the turbulence of the gas, must be in slow overall contraction towards the nucleus. The existence of the central cavity may require a recent (≊105 yr ago) energetic nuclear event.

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