Giant molecular clouds and massive star formation in the southern milky
  way
  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(H2) = 1.14 +/-
0.05 x 10^8 Msolar accounting for around 40% of the molecular mass estimated
from an axisymmetric analysis of the H2 volume density in the Galactic disk
(Bronfman et al. 1988b) M(H2) disk = 3.03 x 10^8 Msolar. 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{\deg}, 330{\deg}, and 338{\deg}, 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 H2
mass (MSFR), and the massive star formation efficiency for GMCs. The average
MSFR for GMCs is 0.41 +/- 0.06 Lsolar/Msolar, and for the most massive clouds
in the Norma arm it is 0.58 +/- 0.09 Lsolar/Msolar. Massive star formation
efficiencies of GMCs are on average 3% of their available molecular mass.
