2010ApJ...709..424M

We use the Wilkinson Microwave Anisotropy Probe (WMAP) maximum entropy method foreground emission map combined with previously determined distances to giant H II regions to measure the free-free flux at Earth and the free-free luminosity of the Galaxy. We find a total flux f_ν= 54, 211 Jy and a flux from 88 sources of f_ν= 36, 043 Jy. The bulk of the sources are at least marginally resolved, with mean radii ∼60 pc, electron density n_e∼ 9/cm3, and filling factor φ_H ii~0.005 (over the Galactic gas disk). The total dust-corrected ionizing photon luminosity is Q = 3.2x10⁵³±5.1x10⁵² photons/s, in good agreement with previous estimates. We use GLIMPSE and Midcourse Space Experiment (MSX) 8 µm images to show that the bulk of the free-free luminosity is associated with bubbles having radii r ∼ 5-100 pc, with a mean of ∼20 pc. These bubbles are leaky, so that ionizing photons emitted inside the bubble escape and excite free-free emission beyond the bubble walls, producing WMAP sources that are larger than the 8 µm bubbles. We suggest that the WMAP sources are the counterparts of the extended low density H II regions described by Mezger. The 18 most luminous WMAP sources emit half the total Galactic ionizing flux. These 18 sources have 4x10⁵¹/s ≲ Q ≲ 1.6x10⁵²/s, corresponding to 6x10⁴ M_☉≲ M_*≲ 2x10⁵ M_☉; half to two thirds of this will be in the central massive star cluster. We convert the measurement of Q to a Galactic star formation rate (SFR) {dot}M_*=1.3±0.2 M_☉ yr^-1, where the errors reflect only the error in free-free luminosity. We point out, however, that our inferred {dot}M_* is highly dependent on the exponent Γ≈ 1.35 of the high-mass end of the stellar initial mass function. For 1.21 < Γ < 1.5, we find {0.9 M_☉ yr^{-1}<{dot}M_*<2.2 M☉ yr^{-1}}. We also determine a SFR of 0.14 M_☉/yr for the Large Magellanic Cloud and 0.015 M_☉/yr for the Small Magellanic Cloud.