The Near and Far 3-Kpc Arms in CO Emission

The two parallel lanes marked by the flashing red lines might look like a set of muddy tire tracks to you, but to Galactic astronomers they represent some of the first direct evidence that the Milky Way has the sort of two-fold symmetry expected of a barred spiral galaxy.

This diagram displays the distribution of dense molecular gas (as traced by CO emission) as functions of angle along the Galactic plane and radial velocity. The parallel lanes of emission are believed to be two similar spiral arms driven by the ends of the Galactic bar. The lower, brighter arm was discovered over 50 years ago and dubbed the "Expanding 3-Kpc Arm" because of its expansion velocity of –53 km/s at longitude 0°. Its more distant and elusive counterpart was discovered only recently and called the "Far 3-Kpc Arm". Both can be seen in this recent artist's illustration of the Milky Way.