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21 cm Line Emission

Spin flip

Neutral hydrogen (HI) exists in abundance in interstellar space and consists of one proton and one electron. Hydrogen not only has energy levels associated with its electron's orbit but also with the orientation of the electron's spin. When the electron and proton are spinning in the approximate parallel sense, poles aligned, the atom has a slightly higher energy than when the spins are approximately anti-parallel. An atom always wants to exist in its lowest energy state so if the electron has been knocked to the higher parallel state it will decay to the lower state emitting a 21 cm photon. The energy levels are fine (very small) so the average decay half-life is 12 million years, although this is reduced to 400 years in hydrogen clouds where collisions are common. This may seem like a long time but there are generally enough atoms in a cloud to allow a significant detection.

The fine structure energy can be predicted through quantum mechanics. Consider the electron orbiting the nucleus, now change to the rest frame of the electron. In this frame the positive nucleus orbits the electron - generating a magnetic field (B). The electron has an associated spin and so magnetic moment.


Where s is a half. Quantum mechanics predicts that the spin magnetic moment can have only two possible orientations to the magnetic field, spin up and spin down. The energy deviation from the gross structure energy of the hydrogen atom is:


And so the energy associated with the transition between the two spins is twice this.

Past 21 cm surveys:- The Westerbork Synthesis Radio Telescope (WSRT)

The WSRT is a large array telescope, consisting of 23 dishes, built in the Netherlands during the 1960s. It was originally designed to observe at 21 cm, however each dish was constructed to such accuracy that it can now observe at even shorter wavelengths, notably 18 cm. It was the brainchild of Professor Jan Oort who desired a radio telescope with a resolution that would allow the structures of extragalactic sources to be resolved. The WSRT was the first radio telescope to detect and map the spiral arms of another galaxy, M 51.