The Columbia - U. de Chile CO Survey of the Southern Milky Way is used for separating the CO ( 1-0 ) emission of the fourth Galactic quadrant within the solar circle into its dominant components , giant molecular clouds ( GMCs ) .
After the subtraction of an axisymmetric model of the CO background emission in the inner Southern Galaxy , 92 GMCs are identified , and for 87 of them the two-fold distance ambiguity is solved .
Their total molecular mass is M ( H _ { 2 } ) = 1.14 \pm 0.05 \times 10 ^ { 8 } M _ { \odot } accounting for around 40 % of the molecular mass estimated from an axisymmetric analysis of the H _ { 2 } volume density in the Galactic disk ( 14 ) M ( H _ { 2 } ) _ { disk } = 3.03 \times 10 ^ { 8 } M _ { \odot } .
The large scale spiral structure in the Southern Galaxy , within the solar circle , is traced by the GMCs in our catalog ; 3 spiral arm segments : the Centaurus , Norma , and 3-kpc expanding arm are analyzed .
After fitting a logarithmic spiral arm model to the arms , tangent directions at 310 ^ { \circ } , 330 ^ { \circ } , and 338 ^ { \circ } , respectively , are found , consistent with previous values from the literature .
A complete CS ( 2-1 ) survey toward IRAS point-like sources with FIR colors characteristic of UC HII regions is used to estimate the massive star formation rate per unit H _ { 2 } mass ( MSFR ) , and the massive star formation efficiency ( \epsilon ) for GMCs .
The average MSFR for GMCs is 0.41 \pm 0.06 L _ { \odot } /M _ { \odot } , and for the most massive clouds in the Norma arm it is 0.58 \pm 0.09 L _ { \odot } /M _ { \odot } .
Massive star formation efficiencies of GMCs are on average 3 % of their available molecular mass .