The dominant component in the ISM is hydrogen.
Atomic hydrogen (HI) it emits radiation at the radio frequency of 1420 MHz.
Transition between the two hyperﬁne states of the ground state.
Their energy difference comes from the interaction of the magnetic moments of the proton and electron of the atom.
The most important observational tool to track the orbital motions of clouds of gas about the center of our Galaxy and in the gravitational field of other galaxies.
The rotational curve obtained by these observations is generally is quite ﬂat: evidence for dark matter.
The coupling of angular momenta and the energy level splitting.
Electron spin angular momentum,
Electron orbital angular momentum,
Total electronic angular momentum, .
The hyperfine splitting of the ground state is the source of the 21 cm hydrogen line.
This wavelength falls within the microwave radio region.
Fine and hyperfine structure in hydrogen. Credit: Wikipedia Commons.
The energy difference between the two states is small:
According to formula , we obtain the value of 1420 MHz for the frequency of the emitted radiation.
The magnetic moment of an electron is opposed to its spin: therefore, the more stable state has opposed spins.
Effect of collisions.
The kinetic energies in ISM: even in the coldest regions at temperatures of about 100K, we .
HI images of nearby spiral galaxies. Credit: NRAO/AUI and F. Walter, Max Planck Institute for Astronomy.
The Magellanic Stream is an extended HI stream encircling the Milky Way. Credit: Nidever et al. (NRAO).
Splitting of atomic energetic levels in presence of a magnetic ﬁeld.
When a static magnetic ﬁeld is applied, each of the ﬁne-structure levels splits into 2J +1 energy levels, with energies depending on the value of .
Interstellar magnetic ﬁeld strengths: ,
Hyperﬁne splitting in atomic hydrogen: 21-cm transition.
Detection of the effect
Application to HI regions.
Diagram of energetic level under a magnetic field. Credit: Wikipedia Commons.
Zeeman effect observations are useful in studying the interstellar magnetic field.
Advantages and disadvantage of Zeeman effect measurements.
Aperture synthesis techniques.
Zeeman effect in radio frequency spectral lines provides a strict lower limit on the magnetic field.
Rotation of the linear polarization vector of light which occurs when polarized radiation passes through a magnetized and ionized medium.
Radio observations: dispersion and polarization rotation the mean of the magnetic field along the line of sight can be measured.
Application to magnetic fields in the ISM.
Verschuur & Kellerman, “Galactic and Extragalactic Radio Astronomy”
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