The radial distribution of OB star formation in the Galaxy

We present the azimuthally averaged radial distribu- tion of 748 regions of OB star formation in the whole galac- tic disk, based on our previous CS(2-1) survey of UC H II regions. Embedded massive stars produce a total FIR lumi- nosity of 1:3910 8 L within the range 0:2 R=R 2 in galactocentric radius. We find 492 massive star forming regions within the solar circle, producing 81% of the total FIR luminos- ity. Separate analyses of the 349 sources in the I and II quad- rant (north), and of the 399 sources in the III and IV quadrant (south), yield FIR luminosities (extrapolated to the complete galactic disk) of 1:1710 8 L and of 1:6010 8 L, respectively. Massive star formation is distributed in a layer with its centroid Z(R) following that of molecular gas for all galactocentric radii, both north and south. Its thickness for R R is 73 pc (FWHM),62% the thickness of the molecular gas disk. The FIR luminosity produced by massive stars has a well defined max- imum at R =0 :55 R, with a gaussian FWHM of 0:28 R- compared with 0:51 R for the H2 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, compared with 0:34 R for the H2 surface density. Massive star formation per unit H2 mass is maximum for R 0:55 R in the southern Galaxy, with a FIR surface luminosity to H2 surface density ratio of 0:41 L=M, com- pared with 0:21 L=M at the same radius in the north, and with an average of 0:18 L=M for the whole galactic disk within the solar circle.