Carbon monoxide in the inner Galaxy - The 3 kiloparsec arm and other expanding features

A survey of the latitude distribution of (C-12)O emission in the inner Galaxy shows that much of the dense molecular gas is found in several large-scale features spanning tens of degrees in l. The most prominent are the rotating nuclear disk, the 3 kpc arm, and the +135 km/s features. Each of these features is moving with a high (50-180 km/s) noncircular velocity, which can be unambiguously interpreted as expansion away from the galactic center. The (l, v) loci of the 3 kpc arm and +135 km/s feature can be described by simple kinematic ring structures, but the same ring cannot account for both objects.