We normally study galaxies by looking directly at them. We admire the brilliant arms in spiral galaxies like the Milky Way, the giant football-shaped elliptical galaxies, or even the powerful jets emanating from galaxies with supermassive black holes at their centers, the so-called Active Galactic Nuclei (AGN).
As dramatic as such images are, much can be gained also by looking at the impact of these spectacular light-producing objects on the material that sits between the galaxies. We know that hydrogen exists in large amounts in stars and in the galaxies that house the stars. Hydrogen also is the main material found in between the galaxies.
Hydrogen, which is made up of one proton and one electron, has a property that it is opaque to UV radiation. A UV photon of light incident on a hydrogen atom will be absorbed. An effect of this absorption is that the electron will be freed or ‘ionized.’ This ionization leaves a signature in the light we receive at our telescopes in the form of a dark band to mark the region from which the electron was removed.
When we look deep into space extending back to the first one billion years after the Big Bang, we see that the UV light from galaxies and AGN has ionized the intergalactic hydrogen. There are so many overlapping dark bands that it is obvious that hydrogen everywhere is being ionized. By contrast, when we look at galaxies near to us we see far less evidence for these dark bands, which in this case we take as an indication that this material, which should be opaque to UV light, is not doing its job because the atoms are already ionized.
Our best interpretation is that the first galaxies to form in the universe emitted sufficient amounts of UV radiation to escape their own galaxies and to ionize hydrogen atoms in intergalactic space. After a continual bombardment of this intergalactic hydrogen for hundreds of millions of years, all of the hydrogen between the stars eventually became ionized.
Upon reaching this time in cosmic history which we call the epoch of reionization, the regions between the stars remained completely ionized continuing on to this day. There are many uses for this humble approach to study galaxies. For example, we hope to use this technique to help us ‘age-date’ when the first stars actually did form, a primary goal of the upcoming James Webb Space Telescope satellite (launch in 2018).