How do we know there are intergalactic clouds between a distant quasar and earth?

Question

Claire Anctil · Accepted Answer

We know this because of the Lyman-alpha forest.

As light from a distant quasar passes through clouds of intergalactic gas, the gas will absorb light at very specific wavelengths, including the Lyman alpha line of hydrogen at 122 nm (in the ultraviolet).

Since each one of these clouds will be at a different redshift, with the closest clouds having the smallest redshift and the furthest clouds having the largest redshift, you will see a whole bunch of absorption lines at slightly different wavelengths. In fact, there are so many absorption lines, it looks like a "forest".

A simplified diagram:

Actual data:

The big hump on the right is the Lyman alpha emission line from the quasar. All of the other lines are the "Lyman-alpha forest". Each one of the absorption lines (dips) is a Lyman alpha absorption line from a different cloud of gas that light from the quasar has passed through. The smaller the wavelength, the close the cloud is to us.

Ivy Allen · Answer

undefined Because of absorption lines seen in the spectrum of a distant quasar, which are produced when light passing through gas clouds on its way to Earth is absorbed, we know that there are intergalactic clouds between Earth and the quasar. By examining these absorption lines, scientists can ascertain the characteristics and makeup of the intergalactic clouds.

Ariana Acosta · Answer

undefined We know there are intergalactic clouds between a distant quasar and Earth because the light from the quasar passes through these clouds on its way to us. As the light travels through the intergalactic clouds, it gets absorbed by the atoms and molecules present in the clouds, leaving behind absorption lines in the quasar's spectrum. By analyzing these absorption lines, astronomers can determine the composition, density, and other properties of the intergalactic clouds. This phenomenon is known as the Lyman-alpha forest and provides valuable insights into the distribution of matter in the universe.