The Radio Universe
[Astronomy]  [Topics]

(1) HII Regions:

  The space between the stars is not empty: it consists of gas and dust and is full of electromagnetic radiation. Each cm of interstellar space contains an average of 1-2 H atoms. The ratio of gas to dust, by mass, is estimated to be about 100:1.

Clouds of neutral Hydrogen (ie. normal H, with one proton and one electron) are referred to as HI regions. These exist throughout the galaxy, but particularly when well away from hot stars (T > 20000K). The continuum radiation from a hot star contains many UV photons and can ionise the H gas. Cooler stars cannot provide enough UV and the H gas remains neutral.

So clouds of H gas near to hot stars will tend to be ionised (there will be some neutral H as well), and such an area of space is called an HII region. These regions are particularly associated with O and B type stars which can supply plenty of UV photons with E > 13.6eV, the ionisation energy of the H atom.

  An HII region thus consists of many H ions (protons) and free electrons. When the protons capture free electrons a continuum of radiation is produced, as the free electrons can have any energy (their average energy is determined by the temperature of the gas cloud). UV, visible and IR radiation is produced as captured electrons cascade down the H energy levels. It is often the case that little of the UV radiation provided by the star, that produces the HII region in the first place, escapes: it is mostly "degraded" to lower energy photons by the gas. In particular, HII regions emit very strongly in the visible: the Balmer series (n = m to n = 2 transitions) is very evident, with n = 3  n = 2 dominant (the H_a line at ~ 658nm). HII regions thus appear a reddish colour when photographed.
 
 

But the HII region does not only emit these shorter wavelength photons. If a free electron passes near to a proton but does not recombine a so-called free-free transition can take place, with the emission of photons. Since the electron and proton can have an infinity of trajectories, a full range of photon energies, ie. a continuum is produced. And these photons are emitted predominantly in the IR and Radio regions.
 

Since the energy of the free electrons undergoing these "collisions" is governed by the temperature of the HII gas cloud (just like molecules in a gas: Kinetic Theory), HII regions are referred to as THERMAL SOURCES and the radiation produced is called Thermal Radio Emission.

(2) Determining the structure of the Milky Way galaxy using 21cm radiation:
 
 

Hydrogen is by far the most abundant atom in the universe. The hydrogen atom itself is a source of radio emission, due to the hyperfine splitting of its ground state: there is a small difference in energy (5.9 x 10^-6 eV) for a neutral H atom in its ground state depending on whether the proton and electron spins are aligned or not. When the atom changes from one state to another a radio photon with l = 0.21m is produced. The diagram below summarises the situation.
 
 

• The galaxy is full of hydrogen, and away from hot stars it is in its neutral state: the HI regions. Consequently the emission of radio waves by these areas allows us to map out just where the HI regions are. And since there will be plenty of H, the emission will be strong.
• More important still, the emission occurs at a precise wavelength. Any shift in the measured wavelength of the 0.21m radio waves is indicative of a Doppler effect due to the motion of the gas clouds. This allows the measurement of the movements of HI gas clouds within the galaxy to be made.
• Because of their long wavelength, radio waves such as the 0.21m radiation, are unaffected by the dust grains that occupy interstellar space. With visible light, we can only see objects within a distance of about 1kpc (3000ly): this is only about 10% of the distance to the Galactic Centre. However, radio waves that originated in the centre of the Milky Way, and even on its far side, can reach us. So radio waves provide us with a very powerful weapon for mapping the structure of our Galaxy.

In the early 1960s several sources of strong radio emission were found which did not appear to coincide with any of the then-known radio emitting objects, such as galaxies. Optical photographs of these sources revealed them to be faint, starlike objects. One of the brightest of these sources, known as 3C273 (m = 12.8), showed a spectrum consisting of broad emission lines, but the lines were not of any known element. The breakthrough came when it was realised that the lines were ordinary Hydrogen emission features (from the Balmer series) that had been very strongly red-shifted due to the Doppler effect.
 
 

 The received spectrum has been deduced using a value of Dl/l = 0.2, ie. the spectral lines have had their wavelengths increased (red-shifted) by 20%.
 
 

Our Doppler effect equation gives us:
 

The high red-shifts therefore imply a very high recessional velocity and from Hubble's law the objects must be very distant. These objects are called Quasars.
 
 

Some quasars are receding from us with much greater velocities. As an example, the quasar
PKS 2000 -330 shows a value of Dl/l = 3.78. This is so large that the transition from n = 2 to n = 1 in Hydrogen (DE = 10.2eV ) the so-called Lyman aline at 121.6nm in the UV is Dopplershifted into the visible !! (Dl = 3.78l = 460nm, so the observed l = 582nm). Using the proper relativistic equation for the Doppler shift implies that this object is receding from us at 92% of the velocity of light !!
 
 

Quasars are observed to fluctuate in optical brightness, some of the changes occurring in periods of time of the order of days. If the brightness changes are produced by the whole object undergoing a change, the time taken for the change can be no greater than the time it would take a light ray to cross from one extreme edge of the object to the other. Thus the brightness fluctuations indicate that quasars are relatively small, perhaps of the size of our Solar System.
 
 

This creates a major problem. Using the measured values of the apparent magnitude (m) with the distance (d) obtained from Hubble's law the absolute magnitude (M) can be determined. When this is done, M is comparable to that obtainedfor an entire galaxy, yet the energy is being produced in a volume of space comparable to the Solar system !!

The explanation of this puzzle has not, to date, been forthcoming.

Quasars are not particularly bright in the optical region and were detected using radio telescopes. Their detection owes much to the high sensitivity of the detectors used in radio astronomy.