Photoionized mixing layer models of the diffuse ionized gas
  It is generally believed that O stars, confined near the galactic midplane,
are somehow able to photoionize a significant fraction of what is termed the
"diffuse ionized gas" (DIG) of spiral galaxies, which can extend up to 1-2 kpc
above the galactic midplane. The heating of the DIG remains poorly understood,
however, as simple photoionization models do not reproduce the observed line
ratio correlations well or the DIG temperature. We present turbulent mixing
layer models in which warm photoionized condensations are immersed in a hot
supersonic wind. Turbulent dissipation and mixing generate an intermediate
region where the gas is accelerated, heated and mixed. The emission spectrum of
such layers are compared with observations of Rand (ApJ 462, 712) of the DIG in
the edge-on spiral NGC2363. We generate two sequence of models that fit the
line ratio correlations between [SII]/H-alpha, [OI]/H-alpha, [NII]/[SII] and
[OIII]/H-beta reasonably well. In one sequence of models the hot wind velocity
increases while in the other the ionization parameter and layer opacity
increases. Despite the success of the mixing layer models, the overall
efficiency in reprocessing the stellar UV is much too low, much less than 1%,
which compels us to reject the TML model in its present form.
