The Neutral Hydrogen sky is now available on Chromscope. This all came about because Chris Samuel asked whether it could included, and pointed towards an October 2016 press release from the “HI4PI” team. The data that the team made openly available includes a HEALPix map, which makes it straightforward (well, when you’ve done it before) to plot to a full-sky Cartesian map, apply a colour, and divide into tiles.
The image shows the Hydrogen line emission – radio waves emitted by cold hydrogen gas at a wavelength of 21cm (a frequency of around 1.4 GHz). Since the gas is neutral, it must be cold, since if it were heated up to more than a few thousand degrees the gas would be ionised. Ionised hydrogen gas emits light in the visible part of the spectrum, particularly at a specific wavelength called the “Hydrogen alpha” (also available in Chromoscope – giving a nice way to compare the warm and cold parts of our galaxy.