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 degrees the far 3 kpc arm can be followed over at least 20 degrees of Galactic longitude as a faint lane at positive velocities running parallel to the near arm. The far arm crosses l = 0 degrees 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 line widths (similar to 21 km s(-1)), linear scale heights (similar to 103 pc FWHM), and H-2 masses per unit length (similar to 4.3 x 10(6) M-circle dot 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.