The rotation curve for the IV galactic quadrant , within the solar circle , is derived from the Columbia University - U. de Chile CO ( J=1 \to 0 ) survey of molecular gas .
A new sampling , four times denser in longitude than in our previous analysis , is used to compute kinematical parameters that require derivatives w/r to galactocentric radius ; the angular velocity \Omega ( R ) , the epicyclic frequency \kappa ( R ) , and the parameters A ( R ) and B ( R ) describing , respectively , gas shear and vorticity .
The face-on surface density of molecular gas is computed from the CO data in galactocentric radial bins for the subcentral vicinity , the same spectral region used to derive the rotation curve , where the two-fold ambiguity in kinematical distances is minimum .
The rate of massive star formation per unit area is derived , for the same radial bins , from the luminosity of IRAS point-like sources with FIR colors of UC H II regions detected in the CS ( J=2 \to 1 ) line .
Massive star formation occurs preferentially in three regions of high molecular gas density , coincident with lines of sight tangent to spiral arms .
The molecular gas motion in these arms resembles that of a solid body , characterized by constant angular velocity and by low shear and vorticity .
The formation of massive stars in the arms follows the Schmidt law , \Sigma _ { MSFR } \propto [ \Sigma _ { gas } ] ^ { n } , with an index of n = 1.2 \pm 0.2 .
Our results suggest that the large scale kinematics , through shear , regulate global star formation in the Galactic disk .