Diffuse ionized gas in spiral galaxies and the disk-halo interaction

TL;DR

This paper discusses the properties and significance of the warm ionized medium in spiral galaxies, highlighting its role in disk-halo interactions and the implications for interstellar medium models.

Contribution

It provides insights into the structure, ionization, and energy transport mechanisms of the warm ionized medium in spiral galaxies.

Findings

WIM extends far into the halo, indicating disk-halo interaction.

Ionization is propagated through the disk despite neutral hydrogen clouds.

WIM properties differ from classical H II regions, affecting ISM models.

Abstract

Thick layers of warm, low density ionized hydrogen (i.e., the warm ionized medium or WIM) in spiral galaxies provide direct evidence for an interaction between the disk and halo. The wide-spread ionization implies that a significant fraction of the Lyman continuum photons from O stars, produced primarily in isolated star forming regions near the midplane and often surrounded by opaque clouds of neutral hydrogen, is somehow able to propagate large distances through the disk and into the halo. Moreover, even though O stars are the source of the ionization, the temperature and ionization state of the WIM differ significantly from what is observed in the classical O star H II regions. Therefore, the existence of the WIM and observations of its properties provide information about the structure of the interstellar medium and the transport of energy away from the midplane as well as place significant constraints on models.