CAN THE LYMAN CONTINUUM LEAKED OUT OF H II REGIONS EXPLAIN DIFFUSE IONIZED GAS?

We present an attempt to explain the diffuse H alpha emission of a face-on galaxy M 51 with the standard photoionization model, in which the Lyman continuum (Lyc) escaping from H II regions propagates large distances into the diffuse interstellar medium (ISM). The diffuse H alpha emission of M 51 is analyzed using thin slab models and exponential disk models in the context of the on-the-spot approximation. The scale height of the ionized gas needed to explain the diffuse H alpha emission with the scenario is found to be of the order of similar to 1-2 kpc, consistent with those of our Galaxy and edge-on galaxies. The model also provides a vertical profile, when the galaxy is viewed edge-on, consisting of two-exponential components. However, it is found that an incredibly low absorption coefficient of kappa(0) approximate to 0.4-0.8 kpc(-1) at the galactic plane, or, equivalently, an effective cross section as low as sigma(eff) similar to 10(-5) of the photoionization cross section at 912 angstrom is required to allow the stellar Lyc photons to travel through the Hi disk. Such a low absorption coefficient is out of accord with the properties of the ISM. Furthermore, we found that even the model that has the diffuse ionized gas (DIG) phase only and no Hi gas phase shows highly concentrated H alpha emissions around H II regions, and can account for only less than or similar to 26% of the H alpha luminosity of the DIG. This result places a strong constraint on the ionizing source of the DIG. We also report that the H alpha intensity distribution functions not only of the DIG, but also of H II regions in M 51, appear to be lognormal.