Question

How is cool, neutral hydrogen gas, H I, detected in the spiral arms of galaxies?

Answer 1

By identifying spin flip transitions.

Explanation:

Spin flip transitions are a relatively small energy transition, and to understand spin flip, we need to know a few things about hydrogen atoms, and the electromagnetic force.

Atoms are held together by the electromagnetic force. In a hydrogen atom, a single, positively charged proton, attracts a single negatively charged electron. The rules of quantum mechanics dictates that the electron can only orbit the proton at specific energy levels.

Any transition between these energy levels will result in the emission or absorption of a photon. For hot hydrogen, like the hydrogen in Orion nebula, an emission spectrum can be observed. In the case of hydrogen in the atmosphere of stars, an absorption spectrum can be seen.

Notice that both spectra are the same, the only difference is whether light is emitted or absorbed. The wavelength of the `Halpha` photon (the red line) is about 656 nm.

Unfortunately, cold hydrogen is already in its lowest energy level, and there is often no background light source for absorption. Luckily there is one more transition that we can observe.

According to quantum mechanics, subatomic particles have a property called spin. At its most basic, we can picture spin as a rotating top. The top can spin in one of two directions, either left or right. Since electrons and protons carry charge, their rotations will generate small magnetic fields.

These two fields can either be aligned with each other or counter aligned. If they are aligned, the atom will be in a stable equilibrium, meaning that if the atom experiences a small deviation, it will tend to reset. If, however, the particles are counter aligned, a small deviation will tend to cause the electron to "flip" its spin, and the fields will become aligned.

Just like the other spectra, this is an energy level transition, and will result in the emission of a photon. The wavelength of this photon is 21 cm, on the order of a million times the wavelength of the visible emission spectrum. This puts it in the radio region of the electromagnetic spectrum.