Lyman Break --
There are times when I think that the universe is just screaming out "Discover me, discover me!" And to me, this is one of those times. The Lyman Break in a spectrum just seems like an almost 'too good to be true' effect in cosmology because it allows us to constrain the age of galaxies in the universe.
When we are viewing the universe there are only really three things we need to look for. They are:
As we're using redshift to actually determine the age of the galaxies in question, then the Lyman break must either be an absorption or an emission.
The Lyman break happens at a wavelength of 91.1267 nm which is in the extreme ultraviolet part of the spectrum and it is the highest energy photon that a hydrogen atom can emit. What this means for us is that there is a sharp cut off in energy release. Now you might be thinking... There are more energetic photons that are going to be released by the stars within that galaxy, Helium atoms, for instance, are still able to emit photons at higher energies than the ~91 nm.
It then comes down to the following two reasons:
1) Relative abundance of hydrogen vs Helium.
To start with reason one. We know that our sun, stars in general, and the universe as a whole mostly consists of hydrogen. As most of the stars photosphere is made of hydrogen, it is the strongest emitter (and therefore) the strongest source that we're going to detect. Yes, helium in those stars is emitting photons of a higher energy. BUT hydrogen emission is stronger.
The other reason comes down to hydrogen, once again, but for a different reason. Namely, absorption. As the Lyman break is the highest energy photon that can be emitted by a hydrogen atom, any photon of a higher energy will still be absorbed by an electron around a hydrogen atom and would fully ionize it with a little bit of energy to spare. This also leads to an even stronger break because photons higher are being taken away.
It is both of these reasons combined that lead to the really strong break and it is why we call these galaxies Lyman-Break Galaxies.
I'll leave some of the the questions for someone else to answer 🙂
For further reading: