Fundamental radiation processes

Although astronomy is the discipline dealing with objects at the largest distances that we can measure, and thus the very largest spatial scales imaginable, the creation of light received (and investigated) by us occurs on microscopic scales, at the level of molecules, atoms or elementary particles therein. Therefore, in order to gain a basic understanding of what kind of light reaches us from the depth of space, we need to have a very brief look at some basic physics first (not to worry - I won't go into detail).

Line emission

According to the laws of quantumphysics some types of particles are only allowed to have certain, fixed levels of energy (but not in between these). One can compare this with climbing stairs or a ladder - one has a stable footing only at fixed levels, but not in between).

Once we accept that this is the case, it has the natural consequence that such an elementary particle can also absorb or emit only very specific amounts of energy when making a transition from one allowed energy state to another (in our example of scaling a ladder this means that each time we climb one rung we must invest an exactly defined amount of energy, the exact amount of which is released again when we descend).

An elementary particle has two principle ways of absorbing or emitting energy. One of them is to either emit or absorb a photon. The photon will carry exactly the amount of energy corresponding to the difference in allowed values between which the transition takes place (meaning: if we don't push hard enough, we won't climb a rung; but pushing a bit harder won't give us a stable footing in between two rungs). So the energies must match! The particle will absorb a photon only when its energy will bring it exactly to the next higher allowed state; and it will emit only a photon that releases exactly the amount of energy set free by relaxing to the next lower allowed energy state.

Imagine a large number of tiny particles, say electrons, that have in the past been heated up (or "excited") by a nearby energy source, such as a star. But now they have moved away and their surroundings become a bit cooler and suddenly the energy they carry is no longer in equilibrium with their environment. Physical laws rule that such particles will sooner or later release that surplus energy to the environment in order to reach equilibrium again. Statistically, this will often happen over long times. But in outer space, although it might appear to be almost empty, there are in fact an enormous number of such particles. When they release their surplus energy, one can pick up the light (photons) emitted by them if only enough emit light at the same time. Since all emit their photon at the same energy when undergoing the same transition (under the same circumstances), one will see lots of photons coming our way, all with (almost...) the same energy. Now what does that look like in a plot? If we draw a sketch, this means that over a range of energies we see no light coming our way, only at the energy of the transition the particles are undergoing, there will be lots (see figure below). That peak is what is called an "emission line". The process of emitting such light is accordingly called "line emission". It is as easy as that! Now if you have lots of particles moving around at different speeds, you will not measure just one narrow spike, but the emission line will have substructure reflecting the various particle motions. This is what astronomers use to tell how celestial objects are moving. In the image below, all the HI line emission of a rotating spiral galaxy is captured.

Spectrum of the total HI emission of an external galaxy. The width and shape of the emission line are caused by the rotation of the galaxy disk and the distribution of gas in it.

There are, of course, many types of atoms and molecules in the interstellar gas of galaxies. So many more emission lines exist outside the displayed range of energies (wavelengths). Our observing bandwidth being limited, they cannot be investigated all at the same time. However, many (actually most) of them are weaker than the HI line and a good many emission lines cannot be studied in detail, because the signals we receive are too weak.

Note that in the process of being heated up (our starting point in the example above) particles absorb energy/photons and thus one will find, when looking at the particles, photons with the energy they absorb to be "missing". This is what one calls an "absorption line". In the plot above, the line would then not stand up, but show up as a trough.

The technical process employed to measure astronomical line emission is called "spectroscopy".

Continuum radiation processes

There are physical quantities that are not "quantized" as the ones leading to line emission. For example, when two particles collide they can do so under all kinds of circumstances (different speeds, angles, different masses of the particles involved). Therefore, the energy that might be transferred in the process from one particle to the other is in no way pre-determined, or quantized. Any value (up to a certain maximum - no particle can have infinite energy!) is allowed. This means that there are no disallowed energy values; photons emitted by such particles may thus have a "continuous" energy distribution - therefore the term "continuum emission". An example is shown below. An emission line, as presented above, is depicted superimposed on continuum emission (which, actually, consists of two types, as we will see further below).

Sketch describing the superposition of continuum and line emission and their separation for measuring line properties.

Let us consider an analogue from our daily life, in which particles exchange energy by bouncing off each other ("collisional excitation"). Question: How does water at the surface of a pot know that the bottom of the pot is hot? Answer: The particles (water molecules) at the bottom are excited (heated up) by the boiler plate and start erratic motions. (Don't we all start moving erratically when getting excited?) This way, they collide with neighbouring water molecules, which in turn collide with others, and so on, until the whole lot of them knows: There's heat coming from below. Water at high temperatures evaporates in the form of steam; other particles produce light when hitting each other - light that we can observe with very sensitive instruments. An astronomical example of continuum emission, in this case continuum radiation from dust in the spiral galaxy NGC 4565, is displayed below.

Image of millimetre wavelength continuum emission from NGC 4565 by Neininger et al. (1996), Astron. & Astrophys.

Thermal emission

Ha! This is an easy chapter to write. The above example of continuum emission from dust is in fact thermal emission. What that means is that the mean energy of the photons emitted is directly proportional to the temperature of the material the emission comes from. The person to formulate this dependence was Max Planck. In the example displayed below the temperature of the emitter is 2000 degrees Kelvin [K].

Sketch of the Planck spectrum of a 2000 K black body. It indicates that only a small fraction of the emitted light arises in the form of optical emission. Most of the photons are emitted in the near-infrared regime.

Synchrotron processes

What kind of radiation might then not be of thermal origin? Our naive comprehension of such processes is limited, because we do not encounter them in everyday life. So let's be a bit inventive and assume that you are an electron. As such you are a charged particle, which means that you cannot just do as you please, but your motions will be controlled by magnetic fields in your surroundings. Moving towards a magnet you actually start pivoting around the magnetic field line capturing you in crazy spins, entering a helical motion.

Sketch of the helical motion of an electron in a magnetic field and its radiation characteristics, if moving at a velocity close to the speed of light (so-called "relativistic" electron).

Stretching your imagination just a bit further, you might be able to figure out what such motions might do to your stomach.... well, an electron doing the same will emit radiation, because the spiralling motion constitutes a constant change of direction, meaning an accelerated motion, during which the particle loses energy - energy that is set free in the form of low-energy light. The escaping radiation has radio wavelengths and the effect of electrons (i.e. particles at almost the speed of light) in a magnetic field emitting such radiation is called the "synchrotron effect". One measures the same radiation, depending on the setup of the experiment at different wavelengths, in particle accelerators in laboratories on Earth.

An example for such synchrotron emission is the radio image of the edge-on spiral galaxy NGC 891 displayed below. All this emission comes from electrons spiralling in magnetic fields. One can discern a thin galaxy disk, seen on edge, and a more extended component, the "synchrotron halo".

Observed 1.4 GHz radio image of the northern edge-on spiral galaxy NGC 891. All the continuum emission seen in the image comes from relativistic electrons (synchrotron continuum emission).

Continuum emission is usually observed using the technique of "imaging". Note that in the specific case of the above image of NGC 891, the image was obtained by an interferometer, which implies special rules for imaging that are different from those of a single telescope. For polarised emission one would perform polarimetry, while time-variable sources would be studied photometrically.