Redshifts and Distances

The quasars have very large redshifts, indicating by the Hubble law that they are at great distances. The fact that they are visible at such distances implies that they emit enormous amounts of energy and are certainly not stars.

The Most Distant Quasars

The following image from the Sloan Digital Sky Survey shows three of the most distant quasars that are presently known. The quasars are the faint red smudges near the head of each arrow. Their redshift parameters are 4.75, 4.90, and 5.00 respectively, which places them at distances of about 15 - 18 billion light years, depending on the value of the Hubble constant. (The most distant quasars discovered as of the year 2003 have redshifts in excess of 6.)

For reference in discussing the distances and redshifts of quasars, this table relates these quantities to look-back times.

Probing the Early Universe
Because the quasars lie at large redshifts, absorption line systems allow the intergalactic medium to be investigated at large look-back times (that is, early in the history of the Universe). Absorption lines for the most distant quasars may allow us to probe the intergalactic medium near the time of galaxy formation. In particular, Lyman-alpha forests at the largest redshifts may represent metal-free clouds of hydrogen and helium that existed early in the Universe, before they formed galaxies.

Quasar Absorption Lines

In addition to emission lines, many quasars exhibit absorption lines. Some of these may originate in the quasar and its surroundings and have the same redshift as the quasar. However, most quasar absorption lines have redshifts smaller than the redshifts for the emission lines. This implies that these absorption lines originate in matter that the light passes through on its journey to us. By studying these lines, it is possible to learn about the intergalactic medium in the early Universe (see the right box). The light coming from the quasar can pass through many clouds having different redshifts, so the same absorption line can appear at multiple wavelengths in the spectrum (with each wavelength corresponding to a different redshift). Such sets of absorption lines in quasar spectra are called absorption systems. They usually are divided into two categories:

Metal-line systems, which are produced when the quasar light passes through galactic disks or halos lying between us and the quasar and is absorbed by metals (elements heavier than helium).

Lyman-alpha systems, which consist of hydrogen lines only and are produced by primordial hydrogen clouds lying between us and the quasar. (Recall that Lyman-alpha denotes the first transition in the hydrogen Lyman series.) A Lyman-alpha system is often called a Lyman-alpha forest, because the Lyman-alpha absorption line of atomic hydrogen is repeated so many times at different redshifts that the resulting lines almost blend together into a "forest" in the spectrum.

These absorption systems are unrelated to the quasar itself. The only role of the quasar is to provide the background illumination that generates the absorption lines in more nearby clouds of gas.

The Position of Lyman-Alpha

The Lyman-alpha line normally occurs at 1216 Angstroms, which is in the far UV. However, because of the expansion of the Universe the Lyman-alpha emission line for the most distant quasars is shifted all the way into the visible or even infrared part of the spectrum. For example, at a redshift of 3 the Lyman-alpha transition occurs in the visible spectrum at 4864 Angstroms (blue-green light) and at a redshift of 5 it is found in the near infrared spectrum at 7296 Angstroms. The Lyman-alpha forest can extend from roughly the position of the quasar Lyman-alpha line to shorter wavelengths that correspond to absorbing clouds with redshifts progressively lower than for the quasar. Since UV light is absorbed strongly by Earth's atmosphere, only in large redshift quasars will the Lyman alpha forest be redshifted to visible wavelengths and be observable from ground-based telescopes. From the examples given above, quasar redshifts of a little less than 3 or greater are required to observe the Lyman-alpha forest from the ground. Observatories above the atmosphere such as the Hubble Space Telescope suffer no such restriction, of course.