  The Wilkinson Microwave Anisotropy Probe (WMAP) free-free foreground emission
map is used to identify diffuse ionized regions (DIR) in the Galaxy (Rahman &
Murray 2010). It has been found that the 18 most luminous WMAP sources produce
more than half of the total ionizing luminosity of the Galaxy. We observed
radio recombination lines (RRLs) toward the luminous WMAP source G49.75-0.45
with the Green Bank Telescope near 1.4 GHz. Hydrogen RRL is detected toward the
source but no helium line is detected, implying that n_He+/n_H+ < 0.024. This
limit puts severe constraint on the ionizing spectrum. The total ionizing
luminosity of G49 (3.05 x 10^51 s^-1) is ~ 2.8 times the luminosity of all
radio HII regions within this DIR and this is generally the case for other WMAP
sources. Murray & Rahman (2010) propose that the additional ionization is due
to massive clusters (~ 7.5 x10^3 Msun for G49) embedded in the WMAP sources.
Such clusters should produce enough photons with energy \geq 24.6 eV to fully
ionize helium in the DIR. Our observations rule out a simple model with G49
ionized by a massive cluster. We also considered 'leaky' HII region models for
the ionization of the DIR, suggested by Lockman and Anantharamaiah, but these
models also cannot explain our observations. We estimate that the helium
ionizing photons need to be attenuated by > ~10 times to explain the
observations. If selective absorption of He- ionizing photons by dust is
causing this additional attenuation, then the ratio of dust absorption cross
sections for He- and H- ionizing photons should be > ~6.

Astrophysics of Galaxies