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 ^ { \circ } the Far 3-Kpc Arm can be followed over at least 20 ^ { \circ } of Galactic longitude as a faint lane at positive velocities running parallel to the Near Arm .
The Far Arm crosses l = 0 ^ { \circ } at +56 km s ^ { -1 } , quite symmetric with the -53 km s ^ { -1 } expansion velocity of the Near Arm .
In addition to their symmetry in longitude and velocity , we find that the two arms have linewidths ( \sim 21 km s ^ { -1 } ) , linear scale heights ( \sim 103 pc FWHM ) , and H _ { 2 } masses per unit length ( \sim 4.3 x 10 { { } ^ { 6 } } M _ { \odot } kpc ^ { -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-H _ { 2 } 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 .