The neutral-gas disk around the galactic center

Abstract

Detailed far-infrared observations of several atomic and ionic fine-structure lines and molecular rotational lines toward the galactic center are discussed. There is an extensive (≥5 pc radius) neutral gas and dust disk of mass ≥10<SUP>4</SUP>M_sun; surrounding the inner, ionized 2 pc of the Galaxy. This disk is seen almost edge-on and is tilted ≡20° relative to the galactic plane. The bright C<SUP>+</SUP> and O<SUP>0</SUP> far-infrared lines and the far-infrared continuum radiation predominantly arise in a photodissociation region between the inner edge of the disk and cooler molecular gas at R ⪆ 4 pc. This photodissociation region is excited by the intense UV radiation from the galactic center. The authors also report the detection of rotational emission lines of highly excited CO and OH molecules. The far-infrared CO emission may come from a small column density of hotter gas excited by shock waves. The dominant motion of the neutral gas disk is rotation about an axis similar to the rotation axis of the Galaxy. The rotation velocity at a radius of 2 pc, corrected for inclination, corresponds to an enclosed mass of (4.8±1)×10<SUP>6</SUP>M_sun;. The dynamics of interstellar clouds in the galactic center as determined by the nuclear mass distribution is examined.