The Ionized Hydrogen

The warm phase of the ionized medium is associated with HII regions where it is photoionized by hot young stars. These regions occupy only about 2-4 % of the total interstellar medium volume; they have a density of about 1 $cm^{-3}$, and a temperature of 6000-12000 K. The emission is mainly in the optical and ultraviolet band, due to the recombination lines of the ionized gas. The warm phase has also been detected in regions distant from star-forming regions in both our galaxy and in external galaxies. This material, called ``Reynolds layer'' in our galaxy, has a temperature of 6000 K and density of 0.3 $cm^{-3}$, and occupies at least 15% of the total volume.
The hot phase of ionized medium has a temperature larger than $10^6$ K and density less than $10^{-2}$ $cm^{-3}$. It is heated by strong shocks resulting from supernova explosions or violent stellar winds. Near the midplane of galaxies it has a relatively small filling factor, above the galactic plane its buoyancy encourages it to flow out through galactic chimneys in the denser phase of the interstellar medium, so that it occupies a much larger fraction of the available volume out of the plane. The sun sits in such a hot phase, in a bubble of plasma with a hydrogen density of about 5 $10^{-3}$ $cm^{-3}$.
The total mass of the ionized medium is only 0,1% of the total mass of the galaxy.

Table 1.1: The five phases of interstellar matter.

Regions		Density ($cm^{-3}$)	T (K)	ISM Mass Fraction
Molecular clouds		$10^3$	10 - 30	40-50%
Neutral medium	cold	1-100	80	40-50%
	warm	0.3-1	6000	4-6%
Ionized medium	warm	0.1 - 1	6000-12000	0.1%
	hot	$10^{-2}$	$10^6$	$\sim0$