In the wide complex profiles of OH, H2CO and CO spectra observed in directions towards the galactic centre, only the features at radial velocities near + 40 km s-1 are generally believed to originate in the molecular clouds nearest the galactic nucleus. The features at other velocities are associated with clouds or spiral features that can be traced over larger ranges of galactic longitude, implying locations which are more distant from the nucleus. In particular, the features near zero velocity have been traditionally associated with molecular clouds within 1-2 kpc of the Sun. However, H2CO observations with high velocity resolution provide evidence that one cloud with velocity near zero is probably near the galactic nucleus.
CO observations of spiral structure and the lifetime of giant molecular clouds
