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
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