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We use the WMAP maximum entropy method foreground emission map combined withpreviously determined distances to giant HII regions to measure the free-freeflux at Earth and the free-free luminosity of the galaxy. We find a total fluxf_\nu=54211 Jy and a flux from 88 sources of f_\nu=36043 Jy. The bulk of thesources are at least marginally resolved, with mean radii ~60 pc, electrondensity n_e ~ 9 cm^{-3}, and filling factor \phi_{HII}=0.005 (over the Galacticgas disk). The total dust-corrected ionizing photon luminosity is Q=3.2x10^{53}photons/s, in good agreement with previous estimates. We use GLIMPSE and MSX 8micron images to show that the bulk of the free-free luminosity is associatedwith bubbles having radii r~5-100 pc, with a mean ~20 pc. These bubbles areleaky, so that ionizing photons from inside the bubble excite free-freeemission beyond the bubble walls, producing WMAP sources that are larger thanthe 8 micron bubbles. We suggest that the WMAP sources are the counterparts ofthe extended low density HII regions described by Mezger (1978). Half theionizing luminosity from the sources is emitted by the nine most luminousobjects, while the seventeen most luminous emit half the total Galacticionizing flux. These 17 sources have 4x10^{51} < Q <1.6x10^{52}, correspondingto 6x10^4M_\odot < M_*< 2x10^5M_\odot; half to two thirds of this will be inthe central massive star cluster. We convert the measurement of Q to a Galacticstar formation rate dM/dt=1.3M_\odot/yr, but point out that this is highlydependent on the exponent \Gamma~1.35 of the high mass end of the stellarinitial mass function.

Star Formation in Massive Clusters via the Wilkinson Microwave Anisotropy Probe and the Spitzer Glimpse Survey