The radial distribution of OB star formation in the Galaxy

We present the azimuthally averaged radial distribution of 748 regions of O B star formation in the whole galactic disk, based on our previous CS(2-1) survey of UC H II regions. Embedded massive stars produce a total FIR lumin osity of 1.3910(8) L. within the range 0.2 less than or equal to R/R-0 less than or equal to 2 in galactocentric radius. We find 492 massive star form ing regions within the solar circle, producing 81% of the total FIR luminos ity. Separate analyses of the 349 sources in the I and II quadrant (north), and of the 399 sources in the III and IV quadrant (south), yield FIR lumin osities (extrapolated to the complete galactic disk) of 1.1710(8) L. and of 1.60 10(8) L., respectively. Massive star formation is distributed in a la yer with its centroid Z(0) (R) following that of molecular gas for all gala ctocentric radii, both north and south. Its thickness for R less than or eq ual to R-0 is similar to 73 pc (FWHM), 62% the thickness of the molecular g as disk. The FIR luminosity produced by massive stars has a well defined ma ximum at R = 0.55 R-0, with a gaussian FWHM of 0.28 R-0-compared with 0.51 R-0 for the H-2 surface density distribution. Toward the outer Galaxy, down from the maximum, the face-on FIR surface luminosity decays exponentially with a scale length of 0.21 R-0, compared with 0.34 R-0 for the H-2 surface density. Massive star formation per unit Hz mass is maximum for R similar to 0.55 R-0 in the southern Galaxy, with a FIR surface luminosity to Ha sur face density ratio of similar to 0.41 L./M., compared with similar to 0.21 L./M. at the same radius in the north, and with an average of similar to 0. 18 L./M. for the whole galactic disk within the solar circle.