What do hydrogen gas surveys reveal about galaxy formation?
In the case of the Milky Way about 10 percent of the matter is gas and 90 percent is stars. In elliptical galaxies there is no gas — there are just old stars. But it’s the exceptions you look at. [The ALFALFA survey, for example, is a project searching for galaxies that consist mainly of hydrogen and dark matter — but not stars. These galaxies could provide essential clues to how galaxies form.] Meanwhile, while we’re looking for the mass of gas, optical telescopes are looking for the composition of stars. It’s these pieces of information in combination that are really valuable for research.
