Encyclopedia

Lyman α

As yet, the only way of receiving information from outer space is through radiation, i.e. light. Radiation consists of light particles, or photons, which are characterized by their energy E, their frequency ν (nu), or their wavelength λ (lambda). The higher the energy, the higher the frequency, but the shorter the wavelength, and vice versa. The different terms are just different ways of talking about the light.

Astronomers often like to measure the wavelength in Ångström (Å), i.e. 100 millionth of a cm. If the wavelength of the radiation lies between approximately 4000 Å and 7000 Å, the light is visible to the human eye. The shorter the wavelength, the more blue the light is, while it becomes more red for longer wavelengths. If λ < 4000 Å, we call it ultraviolet (UV) light, and for even shorter wavelengths we have X-rays and gamma rays. If λ > 7000 Å, we call it infrared (IR) light, microwaves, and radiowaves.

One special type of UV light is particularly interesting to astronomers, namely photons with a wavelength of 1216 Å. Radiation consisting of photons with this wavelength is called Lyman α radiation, or simply Lyα.

Lyα traces hydrogen...

The main reason that Lyα is so interesting is that it is created by hydrogen, and hydrogen makes up 90% of all the elements in the Universe. The energy of a Lyα photon corresponds the energy difference between the ground state and the 1st excited state of the hydrogen atom. This means that a Lyα photon hitting a hydrogen atom in the ground state will excite it to the 1st state. After a while (100 millionth of a second) the atom de-excites back to the ground state, re-emitting the Lyα photon (or another). The photon is said to be scattered.

On the other hand, if a so-called Lyβ photon hits the hydrogen atom, it will excite it to the 2nd state. From here, the atom may de-excite directly to the ground, re-emitting a Lyβ photon, but it may also go first to the 1st state, and then to the ground state, emitting two photons (the sum of energy of which equals the Lyβ photon), in which case the Lyβ photon is destroyed.

Only if the energy of the photon matches very closely the energy difference between the ground and the 1st state, if it is in resonance, will it interact with the atom. Thus, we also say that the Lyα radiation is resonant scattered.

...and hence galaxies

This phenomenon is what makes Lyα so special: it is one of the most commonly produced photons, and even though it is produced deep inside a hydrogen cloud, it may scatter its way through the gas and escape the cloud without being absorbed, making us able to detect the source.

When galaxies form, a large number a maasive stars are formed. The hard UV radiation from these stars ionize the surrounding gas, which recombines and produces an enormous amount of Lyα photons. Also the collapsing gas itself cools by emitting Lyα. On the way out of the galaxy, and through the intergalactic medium, the exact shape of the Lyα spectral line is reshaped according to the physical conditions governing the galaxy. While this makes the interpretation of the line notoriously difficult, it is also an essential key to learning about the galaxies.

For instance, the amplitude and shape of the line is determined by properties such as the galaxy's star formation rate, gas temperature and density, dust contents, as well as turbulence, kinematics, and outflows; quantities that characterize a galaxy.

Since Lyα is in the UV range, although it may travel billions of light years through the Universe, it is almost entirely absorbed by our atmosphere. However, for very distant galaxies the cosmological redshift converts it into visible or infrared light, which easily penetrates the atmosphere. Hence, Lyα is one of our main windows to, in particular, the very distant Universe. Since the very distant Universe also means the very early Universe, observations of Lyα radiation is a very efficient way of learning about how the galaxies formed.

And galaxies are pretty cool.