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Giovanni Covone » 7.Physics of HI clouds

Introduction

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.

Hyperfine structure

The coupling of angular momenta and the energy level splitting.

Electron spin angular momentum, $S$

Electron orbital angular momentum, $L$

Total electronic angular momentum, $J$ .

Term symbol: $^{2S+1}L$

The hyperfine splitting of the ground $^2S$ 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.

21cm radiation

The energy difference between the two states is small: $E = 6 \times 10^{-6} {\rm eV}$

According to formula $\nu = \frac{E}{h}$ , 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 $E_K \sim kT \sim 10^{-2} {\rm eV}$ .

Spectra of compact sources

Emission and absorption spectra towards the compact source 1714-386. Credit: Dickey et al. (1983).

HI in nearby galaxies

HI images of nearby spiral galaxies. Credit: NRAO/AUI and F. Walter, Max Planck Institute for Astronomy.

Rotation curves of spirals

Rotation curve of spiral NGC 2403. Adapted from de Block et al. (2008).

Rotation curves of spirals

Distribution of HI clouds in the M81 group of galaxies. Credit: NRAO/AUI.

Galaxy collision seen at 21cm

A collision between two galaxies, UGC 813 (right) and UGC 816 (left) seen at 21 cm. Credit: NRAO.

The Magellanic stream

The Magellanic Stream is an extended HI stream encircling the Milky Way. Credit: Nidever et al. (NRAO).

The Magellanic stream (cont.)

A 3D view of the Magellanic Stream. Credit: Dallas Parr (CSIRO).

Zeeman effect

Splitting of atomic energetic levels in presence of a magnetic ﬁeld.

When a static magnetic ﬁeld $B$ is applied, each of the ﬁne-structure levels splits into 2J +1 energy levels, with energies depending on the value of $J \times B$ .