A Molecular Face-on View of the Galactic Center Region

Tsuyoshi Sawada1, Tetsuo Hasegawa2, Toshihiro Handa3, and R. J. Cohen4

1 Nobeyama Radio Observatory, 462-2 Nobeyama, Minamimaki, Minamisaku, Nagano 384-1305, Japan
2 National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
3 Institute of Astronomy, University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
4 University of Manchester, Nuffield Radio Astronomy Laboratories, Jodrell Bank, Macclesfield, Cheshire SK11 9DL, U. K.

E-mail contact: sawada@nro.nao.ac.jp

We present a method to derive positions of molecular clouds along the lines of sight from a comparison between 2.6 mm CO emission lines and 18 cm OH absorption lines, and apply it to the central region of the Milky Way. With some simple but justifiable assumptions, we derive a face-on distribution of the CO brightness and corresponding radial velocity in the Galactic center without any help of kinematical models. The derived face-on distribution of the gas is elongated and inclined so that the Galactic-eastern (positive longitude) side is closer to us. The gas distribution is dominated by a barlike central condensation, whose size is about 500pc x 200pc. The major axis of the condensation is tilted with respect to the line of sight by an angle of ~70 deg (tilted by ~40 deg from the large-scale stellar bar). This geometry resembles central regions of barred galaxies. The velocity field shows clear signs of noncircular motion in the central condensation. The so-called ``expanding molecular ring'' feature corresponds to the peripheral region surrounding the central condensation with the Galactic-eastern end being closer to us. These characteristics agree with a picture in which the kinematics of the gas in the central kiloparsec of the Galaxy is under a strong influence of a barred potential.