What is the Lyman α forest?
What is a damped Lyman α absorber?

Quick explanation

If you look at a spectrum of a very bright and very distant light source (usually a quasar), you'll see a lot of "noise", or absorption lines, caused by all the gas clouds lying in between the light source and us. These lines are called the Lyman α forest.

If the line of sight pierces an extra large gas cloud — the seed of a galaxy — an extra strong absorption line will arise; this is called a damped Lyman α absorber.

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The Lyman α forest

When light travels trough the Universe, it gets redshifted. Light that is originally more blue than the wavelength of Lyα, will eventually be redshifted enough to be Lyα. If this happens in a place in the Universe where there happens to be a cloud of neutral hydrogen, some of the light of the original spectrum (e.g. a bright quasar) will be absorbed, and the spectrum will have an absorption line. Since the Universe is full of hydrogen clouds, the spectrum on the blue side of the originally emitted Lyα line may have many absorption lines, and we call it the "Lyα forest" (if the source is very distant, the light can be completely gone, in which case we call it a "Gunn-Peterson trough"). In this way it is possible to map the structure of the intergalactic medium along the line of sight to the background source.

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Lyα forest: Spectrum of a quasar at a redshift z = 3.6. The blue side of the α line is characterized by numerous thin absortion lines.

Damped Lyα absorbers

Sometimes the light will encounter an extremely huge cloud of hydrogen. If the column density NHI is larger than 2×1020 cm-2 (this roughly means if the beam of light encounters more than 12 million atoms on its way through the cloud), then the absorption line becomes so broad that you begin to see the so-called "damping wings" of the line (more absorption far from the center of the line), and hence this is called a damped Lyα absorber (or just DLA). The reason these are interesting is that such massive amount of neutral hydrogen is able to cool sufficiently to make stars, and hence we think that these clouds are the progenitors of present-day galaxies. And galaxies are so fantastic that we would like to know all about them.

If DLAs are really galaxies in their infancy, then in principle it should be possible to detect the light from the first stars. The problem is that the light from the background source usually is so bright that it is hard to see anything else. However, a handful of DLAs has been found where there is a small emission peak in the bottom of the trough. In fact Giorgos, Christa, Johan, and I think we have found one of these (see the picture on the left and judge by yourself). This is emission of Lyα, which is what is expected from star-forming regions.

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Damped Lyα absorber: Spectrum of a quasar at a redshift z = 2.6. On the right is a zoom-in of the DLA region. Perhaps there is a little bit of emission in the bottom. I'm not sure...