External Galaxies

Galaxies external to our home galaxy (the "Milky Way") are interesting and challenging targets for neutral hydrogen line emission observations. In order to be detectable by neutral hydrogen line emissions at the large distances, the galaxies need to be hydrogen rich - usually meaning they are still "stellar nurseries" where dense nebulae are forming stars from molecular H₂ gas and dust enriched with heavier elements. Associated with the H₂ gas (and HII ionised gas) are quantities of atomic neutral hydrogen (HI) gas. It is this form of hydrogen gas that emits the hydrogen line signal at 21 cm.