2012ApJ...749...49R

Murray & Raham used the Wilkinson Microwave Anisotropy Probe (WMAP) free-free foreground emission map to identify diffuse ionized regions (DIRs) in the Galaxy. 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+. This limit puts severe constraint on the ionizing spectrum. The total ionizing luminosity of G49 (3.05x10⁵¹/s) is ∼2.8 times the luminosity of all radio H II regions within this DIR and this is generally the case for other WMAP sources. Murray & Rahman propose that the additional ionization is due to massive clusters (∼7.5x10³ M_☉ for G49) embedded in the WMAP sources. Such clusters should produce enough photons with energy ≥ 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" H II 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.