Star Formation in Massive Clusters via Thewilkinson Microwave Anisotropy Probeand Thespitzerglimpse Survey

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 cm −3 , and filling factor (over the Galactic gas disk). The total dust-corrected ionizing photon luminosity is Q = 3.2 × 10 53 ± 5.1 × 10 52 photons s −1 , 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 4 × 10 51 s −1 ≲ Q ≲ 1.6 × 10 52 s −1 , corresponding to 6 × 10 4 M ☉ ≲ M * ≲ 2 × 10 5 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) , where the errors reflect only the error in free–free luminosity. We point out, however, that our inferred 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 . We also determine a SFR of 0.14 M ☉ yr −1 for the Large Magellanic Cloud and 0.015 M ☉ yr −1 for the Small Magellanic Cloud.