## Lyman Alpha Forest I November 30, 2008

Posted by keithkchan in Cosmology.

In the Astrophysics course, we have to present a paper for the evaluation of the course. I chose to talk about Lyman alpha forest. I am learning a little bit about it now. I would like to make some posts on it.

Lyman alpha forest (BTW, this name is very cool!) refers to the absorption spectrum of distant Quasi Stellar Object (QSO) due to neutral hydrogen atoms in the InterStellar Medium (ISM). There are a lot of names, let me try to make sense of them. QSOs (also called quasar) are very bright objects called Active Galactic Nuclei (AGN) (Oh come on I am introducing more and more terms!), which is believed to a galaxy with a (super)massive black hole at the centre. Because QSOs are very bright, they can be seen at large distance (the most distant QSO is about redshift 6). ISM is the gas and dust that fill the space in between stars and galaxies. Lyman alpha refers to the Lyman alpha transition between n=1 and 2 level in the hydrogen atom. The wavelength is 121.6 nm.

The universe is expanding, and so the photons are stretched and the spectrum is redshifted (the spectrum shifts to the larger wavelength: $\lambda_{\rm obs} = (1 + z) \lambda_{\rm em }$. That is the observed photon with wavelength increased by a factor of 1+z.

This cartoon is from Ned Wright.
On the right, we see a QSO, and the peak corresponds to the Lyman alpha emission from the QSO, and it suffers the largest redshift. In the middle, there are clouds in the ISM which contain neutral hydrogen. These neutral hydrogen atoms absorb Lyman alpha photon, and this corresponds to the troughs in the spectrum. They appear at different wavelength since they have different redshift factor 1+z.

Here is a real spectrum from Bill Keel

The whole bunch of messy lines on the left of the peak is the “forest” of Lyman alpha absorption lines. Note that the horizontal axis is emitted wavelength in the frame of the QSO.