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^-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 (~21 km s^-1), linear scale heights (~103 pc FWHM), and H₂ masses per unit length (~4.3 × 10⁶ M_⊙ 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-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.