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J.B. Dove (MSCD), J.M. Shull (CASA), P.R. Maloney (CASA)
We explore potential solutions to a major puzzle about the diffuse ionized medium (DIM) -- finding an ionization mechanism for H and He that is consistent with both its sources and the observed spectral anomaly in the weak He~I \lambda5876/H\alpha intensity ratio. The most promising mechanism appears to be photoionization by LyC radiation emitted from OB associations; only ~8% of the ionizing photons need escape into the Reynolds layer to sustain the photoionization rate. However, He~I \lambda5876/H\alpha is observed to be much less than that observed in classical H~II regions, requiring the radiation field to be much softer than that expected from OB associations. The inferred ratio of helium ionizing photons to hydrogen ionizing photons, Q1/Q0 \lesssim 0.03 (Reynolds & Tufte 1995, ApJ, 439, L17), is a factor of 7--10 below values obtained from OB association models. Either the intrinsic spectrum of OB associations has Q1/Q0 \lesssim 0.03, or the radiation is softened between the association and the DIM.
The intrinsic spectrum can be this soft if the OB association contains only B stars with T\rm eff \lesssim 35,000 K (Sembach et al.\ 2000, ApJ, 528, 310). The ionizing photon luminosity from such associations would then be much too low to sustain the DIM unless \gtrsim 90% of the emitted photons penetrated into the DIM. Although softening the LyC radiation through reprocessing by cold dense gas has been suggested, we find that this process is inefficient, owing to the relatively high He~I photoionization cross section (\lambda \leq 504 Å). Another promising possibility is that the LyC radiation is filtered by small dust grains in the DIM, whose extinction curve increases with decreasing wavelength between the ionization edges of H~I (912 Å) and He~I (504 Å). However, it may prove difficult to obtain an extinction curve that rises sufficiently rapidly at short wavelengths, to provide adequate softening without overly attenuating the total Q0.
This work was supported by the NASA Astrophysical Theory Program at the University of Colorado, grant NAG5-4063.
The author(s) of this abstract have provided an email address for comments about the abstract: dove@cygnus.mscd.edu