Ionized Hydrogen (HII). While ionized hydrogen (protons, electrons) forms the majority of the ionized phase of the ISM, it also contains ionized forms of other elements: e.g., OII, OIII, CIV, MgII.
The top two galaxies display the largest concentration of HII regions and young stars. The galaxy at the bottom has the sparsest collection of HII regions.
NGC 6946 in visible light (left) and HI radio emission (right)
IRAS composite image of interstellar dust in the Milky Way