Lyman-alpha forest: why not Lyman-beta?

In summary: You never quite get all the way to the lower wavelengths because the Lyman alpha emission from the quasar itself is also present at those wavelengths.In summary, the multiple absorption lines in the spectrum of distant quasars are due to the presence of intergalactic hydrogen clouds that absorb light at the wavelenght of 1216 A, the Lyman-alpha transition.
  • #1
astrolollo
24
2
Hello everyone!
We observe the so-called Lyman-alpha forest in the spectrum of distant quasars and it is said that these multiple absorption lines are due to the presence of intergalactic HI clouds that absorb light at the wavelenght of 1216 A , the Lyman-alpha transition. My question is the following. Why are these absorption lines referred only to Lyman-alpha processes and not, for example, to Ly-beta transitions? Thanks to everyone who will answer.
Lorenzo
 
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  • #2
Perhaps because of the intensity ratio between Ly##\beta## and Ly##\alpha## ?
(about a factor 100 less for the solar spectrum -- don't know about distant quasars)
 
  • #3
I don't think this is the case.. The high-z quasar has a continuum emission. A cloud at lower z will not interact with the radiation coming from the Ly-a or Ly-beta emission of the quasar, but with photons that were emitted at a higher energy. Since the cloud should be quite cold (~10 K ?) all the hydrogen is in the fundamental state. As far as I know, if a gas of atoms in the fundamental state is irradiated with light, there shouldn't be any privileged transition . That is, Ly-a is as likely as Ly-beta or Ly-gamma. Am I wrong? Thanks.
 
  • #5
Strong argument. My mistake (switching emission and absorption) is embarassing ... o:)
We need an expert. @Andy Resnick, perhaps ?
PS what about the last figure here ?

[edit] ahhh, BS to the rescue ! To me it looks like a mixed ##\alpha,\beta## forest ?
the picture in the wiki lemma also shows absorption 'peaks' (at the red arrows) -- for much longer wavelengths -- with the same ##\lambda_\alpha/\lambda_\beta## ratio

upload_2017-1-25_16-58-30.png

(Wiki says: Picture ESO)
 
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  • #6
Yes, there should be Lyman beta as well, but the cross section for absorption is much lower, perhaps a factor of about 5 or a little more if I recall correctly. That creates a proportionally less deep absorption line, if the Lyman alpha is not saturated (i.e., not completely dark at line center). It has to do with the quantum mechanical coupling between the ground state and the second and third levels, the former is quite a bit stronger than the latter so you get deeper absorption in Lyman alpha. Still, I imagine that any time you see a Lyman alpha line in the spectrum, and you can infer the hydrogen column producing it, you immediately include the corresponding weaker absorption feature at the wavelength of Lyman beta. If it's not seen in the spectrum, you have a problem! Also, note that since the wavelength ratio is (1-1/4)/(1-1/9) = 27/32, then for any observed quasar at redshift z, you have a "clean" purely Lyman alpha region from that z down to a z of 27/32*(z+1) - 1, which is 27z/32 - 5/32. It seems you could model the Lyman alpha absorption in that region, calculate the corresponding Lyman beta absorption and remove it from the data at lower wavelengths, then what's left at the lower wavelengths is again all Lyman alpha, which you model and remove the Lyman beta at the next lowest series of wavelengths, etc.
 
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1. What is the Lyman-alpha forest?

The Lyman-alpha forest is a series of absorption lines in the spectra of distant galaxies or quasars. These lines are caused by neutral hydrogen gas in the intergalactic medium, and can be used to study the distribution and properties of this gas.

2. Why is it called the Lyman-alpha forest?

The Lyman-alpha forest is named after the Lyman-alpha line, which is the strongest of the absorption lines observed in the spectra. This line corresponds to the transition of an electron from the first excited state to the ground state in neutral hydrogen.

3. Why not use the Lyman-beta line instead of Lyman-alpha?

The Lyman-beta line is also present in the spectra of distant galaxies, but it is much weaker than the Lyman-alpha line. This makes it more difficult to detect and study, and the Lyman-alpha line provides more information about the intergalactic medium.

4. Can the Lyman-alpha forest tell us about the early universe?

Yes, the Lyman-alpha forest can provide valuable information about the early universe. By studying the absorption lines in the spectra of distant galaxies, scientists can learn about the distribution of neutral hydrogen gas and how it has evolved over time.

5. How is the Lyman-alpha forest useful for cosmological studies?

The Lyman-alpha forest can be used to study the large-scale structure of the universe and the effects of dark matter and dark energy. It can also provide insights into the processes of galaxy formation and the role of neutral hydrogen gas in the evolution of galaxies.

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