Star Formation in Massive Clusters via the Wilkinson Microwave
  Anisotropy Probe and the Spitzer Glimpse Survey
  We use the WMAP maximum entropy method foreground emission map combined with
previously determined distances to giant HII regions to measure the free-free
flux at Earth and the free-free luminosity of the galaxy. We find a total flux
f_\nu=54211 Jy and a flux from 88 sources of f_\nu=36043 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 \phi_{HII}=0.005 (over the Galactic
gas 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 8
micron images to show that the bulk of the free-free luminosity is associated
with bubbles having radii r~5-100 pc, with a mean ~20 pc. These bubbles are
leaky, so that ionizing photons from inside the bubble excite free-free
emission beyond the bubble walls, producing WMAP sources that are larger than
the 8 micron bubbles. We suggest that the WMAP sources are the counterparts of
the extended low density HII regions described by Mezger (1978). Half the
ionizing luminosity from the sources is emitted by the nine most luminous
objects, while the seventeen most luminous emit half the total Galactic
ionizing flux. These 17 sources have 4x10^{51} < Q <1.6x10^{52}, corresponding
to 6x10^4M_\odot < M_*< 2x10^5M_\odot; 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 dM/dt=1.3M_\odot/yr, but point out that this is highly
dependent on the exponent \Gamma~1.35 of the high mass end of the stellar
initial mass function.
