Lyman alpha systems are becoming a very useful source of information in physical cosmology.
The Lyman series is the series of energies required to excite an electron in hydrogen from its lowest energy state to a higher energy state. The case of particular interest for cosmology is where a a hydrogen atom with its electron in the lowest energy configuration gets hit by a photon (light wave) and is boosted to the next lowest energy level. The energy levels are given by En = -13.6 eV/n2 and the energy difference between the lowest (n=1) and second lowest(n=2) levels corresponds to a photon with wavelength 1216 angstroms. The reverse process can and does occur as well, where an electron goes from the higher n=2 energy state to the ground state, releasing a photon of the same energy.
The absorption or emission of photons with the correct wavelength can tell us something about the presence of hydrogen and free electrons in space. That is, if you shine a light with wavelength 1216 at a bunch of neutral hydrogen atoms in their ground state, the atoms will absorb the light, using it to boost the electron to a higher energy state. If there are a lot of neutral hydrogen atoms in their ground state, they will absorb more and more of the light. So if you look at the light you receive, intensity as a function of wavelength, you will see a dip in the intensity at 1216 angstroms, depending on the amount of neutral hydrogen present in its ground state. The amount of light absorbed ('optical depth') is proportional to the probability that the hydrogen will absorb the photon (cross section) times the number of hydrogen atoms along its path.
Because the universe has many high energy photons and hydrogen atoms, both the absorption and emission of photons occurs frequently. In Lyman alpha systems, the hydrogen is found in regions in space, and the source for the photons are quasars (also called qsos), very high energy light sources, shining at us from behind these regions.
Because the universe is expanding, one can learn more than just the number of neutral hydrogen atoms between us and the quasar. As these photons travel to us, the universe expands, stretching out all the light waves. This increases the wavelengths lambda and lowers the energies of the photons (`redshifting').
Neutral hydrogen atoms in their lowest state will interact with whatever light has been redshifted to a wavelength of 1216 angstroms when it reaches them. The rest of the light will keep travelling to us.
The quasar shines with a certain spectrum or distribution of energies, with a certain amount of power in each wavelength. At right, the top picture shows a cartoon of how a quasar spectrum (the flux of light as a function of wavelength) might look if there were no intervening neutral hydrogen between the qso and us. In reality, gas around the quasar both emits and absorbs photons. With the presence of neutral hydrogen, including that near the quasar, the emitted flux is depleted for certain wavelengths, indicating the absorption by this intervening neutral hydrogen. As the 1216 wavelength is preferably absorbed, we know that at the location the photon is absorbed, its wavelength is probably 1216 angstroms. Its wavelength was stretched by the expansion of the universe from what it was initially at the quasar, and, if it had continued to travel to us, it would have been stretched some more from the 1216 angstroms wavelength it had at the absorber. Thus we see the dip in flux at the wavelength corresponding to that which the 1216 angstrom (when it was absorbed) photon would have had if it had reached us. As we can calculate how the universe is expanding, we can tell where the photons were absorbed in relation to us. Thus one can use the absorption map to plot the positions of region of intervening hydrogen between us and the quasar. The middle picture at right shows the flux for one nearby region while the bottom picture shows the case for several intervening regions.
It is common to see a series of absorption lines, called the Lyman alpha forest. Systems which are slightly more dense, Lyman limit systems, are thick enough that radiation doesn't get into their interior. Inside these regions there is some neutral hydrogen remaining, screened by the outer region layers. If the regions are very thick, there is instead a wide trough in the absorption, and one has a damped Lyman alpha system. Absorption lines generally aren't just at one fixed wavelength, but over a range of wavelengths, with a width and intensity (line shape) determined in part by the lifetime of the excited n=2 hydrogen atom state. These damped Lyman alpha systems have enough absorption to show details of the line shape such as that determined by the lifetime of the excited state. These dense clumps are thought to have something to do with galaxies that are forming.
One can use the ionization of neutral hydrogen to find galaxies as well, for example the Lyman break systems described in these technical conference proceedings.
There is a very cool visualization of how different systems absorb Lyman-alpha here by Andrew Pontzen.
Uses: We learn several things from these systems, including the following.
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thanks to M. White and L. Hernquist for suggestions and figures.