what is an HI(21cm)source?
H I stands for neutral atomic hydrogen, so I guess this galaxy has had a spectrograph done that found it had concentrations of H I atoms. From doing a brief search on the internet, the 21 cm stands for the line where we would see this element.
These regions do not emit detectable visible light (except in spectral
lines from elements other than hydrogen) but are observed by the 21-cm
(1,420 MHz) region spectral line - Wikipedia
Chemistry is not my forte, so hopefully someone else with more knowledge can give you more info.
by Budgieye moderator
21cm is used in radio astronomy, which uses a longer wavelength of light than optical astronomy. It is a weak radio wave light but it penetrates dust clouds in our galaxy. It is used to calculate the speed at which the object is moving towards us and away from us. It is formed by the flip flop of spin of electrons, not by electron orbital level quantum transitions.
Thanks for the explanations moderators
Astronomers periodic table: http://www.periodicvideos.com/videos/feature_astronomer_table.htm