First atomic hydrogen spectral line images of a nearby galaxyAstronomers have now observed the radio emission from the neutral hydrogen gas (HI) in a nearby galaxy. Hydrogen gas emits radio emission in a spectral line at a very specific frequency of 1420 MHz.
Source: First atomic hydrogen spectral line images of a nearby galaxy (http://www.sciencedaily.com/releases/2012/03/120315094948.htm)