I Lyman Alpha Forest


I'm currently trying to wrap my head around how the Ly-α forest works. This is what I have so far;

A distant quasar produces these Ly-α photons, which occur when a hydrogen's electron drops from n=2 to n=1, and has a wavelength of around 121nm (depending on the exact transition). This light gets redshifted as it travels due to the expansion of the Universe, and eventually it's so redshifted its wavelength has become 21cm. At this wavelength the photon can be absorbed by neutral hydrogen, and so the observed flux of the quasar at this wavelength decreases if the photons happen to be travelling through a cloud of neutral hydrogen at this redshift.

However, where it doesn't make sense to me is the fact there is a forest of these absorption lines. Surely, because the Ly-α line has a specific wavelength, it can only be absorbed by the neutral hydrogen at a specific redshift, not at multiple redshifts?

If the quasar was more like a black body, where it emitted all wavelengths, then this forest would make more sense, as the lower energy light would redshift to the 21cm line sooner than the more energetic photons, and I can see how that would create a forest effect. I cannot wrap my head around this idea if we're only looking at the Ly-α lines that are coming from the quasar.

I will be very grateful for an explanation of what concept I'm missing here.



Staff Emeritus
Science Advisor
Homework Helper
Insights Author
Gold Member
2018 Award
You have misunderstood the Lyman-##\alpha## forest. The Lyman-##\alpha## series is a series of Hydrogen transitions and it is this absorption that is observed in the quasar and galactic spectra. It is not absorption at 21 cm, it is the absorption line of the Lyman-##\alpha## series itself.
So how does this apply to the understanding of the Epoch of Reionisation? I thought there was some connection there, in that the Ly-α forest could tell us when the EoR ends?


Science Advisor
I thought there was some connection there, in that the Ly-α forest could tell us when the EoR ends?
As you observe quasars at higher and higher redshifts, at some point you start noticing that the forest of absorption lines becomes so dense, that it blocks all flux before the Lyman-alpha peak of the emitter (the Gunn-Peterson through). This indicates that there still was sufficiently high optical density of neutral hydrogen at that time - i.e. reionisation had yet to end.
The redshift around which the through disappears is the end of reionisation.

Astrobites has a great article on the subject:
Read part one if the forest itself is still confusing.


Science Advisor
Gold Member
2018 Award
As both Orodruin and Bandersnatch noted above, the Lyman-alpha forest is due to absorption of light by non-ionized hydrogen. More absorption = more neutral hydrogen between us and the emitter. As Hydrogen ionizes, it no longer preferentially absorbs radiation in the Lyman-alpha band (because absorptions in the Lyman-alpha band are caused by the radiation kicking the Hydrogen from its lowest-energy state to the next-higher energy state).

Finally, not mentioned above is the fact that it's a "forest" because the neutral hydrogen gas is located at a range of different redshifts. So the Lyman-alpha absorption line from earlier times gets redshifted, causing absorption across a range of wavelengths.
Ah, I think I get it now.. ignoring the fact I incorrectly thought it was the 21cm line, not the Lyman lines, what I was mostly missing from my understanding was what the spectrum of a quasar looks like. The first plot in the link that Bandersnatch suggested (pt 1) makes it much clearer that the quasar spectrum does in fact cover many wavelengths, which is what allows for the absorption lines at different redshifts. I was thinking that the quasar spectrum was just the largest peak in it and the rest somehow unrelated, but I now understand that peak is from the Ly-α emission line, which is one of many emission mechanisms in the quasar spectrum.
Thank you all, that was very helpful!

Want to reply to this thread?

"Lyman Alpha Forest" You must log in or register to reply here.

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving

Top Threads