APJ LETTERS, ACCEPTED 7/9/08 Preprint typeset using LATEX style emulateapj v. 10/09/06 A NEW SPIRAL ARM OF THE GALAXY: THE FAR 3-KPC ARM

We report the detection in CO of the far-side counterpart of the well-known expanding 3-Kpc Arm in the central region of the Galaxy. In a CO longitude-velocity map at b = 0 ◦ the Far 3-Kpc Arm can be followed over at least 20 ◦ of Galactic longitude as a faint lane at positive velocities running parallel to the Near Arm. The Far Arm crosses l = 0 ◦ at +56 km s−1, quite symmetric with the −53 km s−1expansion velocity of the Near Arm. In addition to their symmetry in longitude and velocity, we find that the two arms have linewidths ( ∼ 21 km s−1), linear scale heights ( ∼ 103 pc FWHM), and H2 masses per unit length ( ∼ 4.3 x 106 Mkpc−1) that agree to 26 % or better. Guided by the CO, we have also identified the Far Arm in high-resolution 21 cm data and find, subject to the poorly known CO-to-H2 ratio in these objects, that both arms are predominately molecular by a factor of 3–4. The detection of these symmetric expanding arms provides strong support for the existence of a bar at the center of our Galaxy and should allow better determination of the bar’s physical properties