Region H I
An HI region is a cloud formed by atomic hydrogen, this can be in a small cold cloud, not very dense and in a non-ionized state, with temperatures around 90 K, or in a large thin, warm cloud with temperatures around 8000 K. Large clouds slowly contract by gravitation to form dense, cold clouds
In astronomical jargon HI is neutral atomic hydrogen, while HII is ionized hydrogen and H2 molecular hydrogen. HI regions do not emit radiation in the visual range, only in the radius region.
The emission is concentrated in a spectral line coming from the superfine transition of hydrogen (between states in which the spin of the electron and the proton is parallel and antiparallel) centered at 1 420 GHz. This hydrogen line is also known as the 21 cm line and is very useful for studying galaxy dynamics. The Tully-Fischer method uses the width of that line to estimate a galaxy's luminosity (which together with a measurement of its brightness allows an estimate of its distance).