Title:Giant Molecular Clouds and Massive Star Formation in the Southern Milky Way.

Abstract:
The Columbia - U. Chile  Survey of the Southern Milky Way is used for separating the CO(1-0) emission, 
within the solar circle,  into the major giant molecular  clouds (GMCs).  A total of 86 GMCs, are identified, 
with a summed molecular mass of M(H2) = 6.02 ± 0.33 10^7 Msolar. These GMCs trace the known spiral 
arms of  Centaurus and Norma, and the 3-kpc expanding  arm. From an updated CS(2-1) survey of IRAS 
point-like sources with FIR colors of UC H II regions,  we derive the massive star formation rate per unit H2 
mass, 0.32 ± 0.05 Lsolar/Msolar, a value  consistent with that  at the peak of the southern molecular ring 
(Bronfman et al. 2000). The massive star formation efficiencies are of  a few percent, suggesting that giant 
molecular clouds are extremely inefficient in forming massive stars.