Can the Lyman Continuum leaked out of H II regions explain Diffuse Ionized Gas?

TL;DR

This study investigates whether Lyman continuum photons escaping from H II regions can explain the diffuse H_alpha emission in galaxy M 51, finding that the required low absorption is inconsistent with typical ISM properties and constraining the ionizing sources of the diffuse ionized gas.

Contribution

It demonstrates that standard photoionization models require unrealistically low absorption coefficients to explain diffuse emission, challenging the Lyman continuum leakage hypothesis.

Findings

Low absorption coefficient needed (~0.4-0.8 kpc^{-1}) is inconsistent with ISM properties.

Model with only DIG phase accounts for less than 26% of H_alpha luminosity.

H_alpha intensity distributions are lognormal for both DIG and H II regions.

Abstract

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 ~ 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 ~ 0.4-0.8 kpc^{-1} at the galactic plane, or, equivalently, an effective cross-section as low as sigma_eff ~ 10^{-5} of the photoionization cross-section at 912A is required to allow the stellar Lyc photons to travel through the H I 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 DIG phase only and no H I gas phase shows highly concentrated H_alpha emissions around H II regions, and can account for only < 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.