when a galaxy first forms? How does the total luminosity of a galaxy change in time? Like stars, galaxies are found in congregations. These congregations are called groups and clusters of galaxies. How do the neighboring galaxies in groups and clusters affect each other?
Astronomers believe that many galaxies went through an extremely energetic early phase of evolution in which almost all of their energy was produced in their centers. This belief is based on the discovery of quasars in the 1960s. On photographic plates quasars resemble stars, yet are as distant as and far brighter than entire galaxies. Evidently, an enormous amount of energy is produced in a tiny volume of space, perhaps as small as our solar system. Importantly, most quasars have been found far away. Since distance translates into time in astronomy, we can infer that most quasars lived and died in the distant past. Quasars are the dinosaurs of the cosmos. Astronomers theorize that quasars constituted the central regions of some galaxies at a very early stage of their evolution.
The new generation of large, visible-light and infrared telescopes and the already launched Hubble Space Telescope may be able to detect the weak light of infant galaxies harboring quasars and to advance the study of the connection between quasars and galaxies. New infrared telescopes, such as SIRTF and the ground-based, infrared-optimized 8-m telescope, will also play important roles in quasar research. Finally, new radio telescopes with extremely high angular resolution, particularly the Very Long Baseline Array, should be able to make radio-wave images of quasars themselves.
Great dust clouds apparently surround many quasars, absorbing their visible light and turning it into infrared radiation. In the 1980s, IRAS discovered extremely luminous galaxies emitting 90 percent or more of their energy as infrared radiation and apparently harboring quasars at their centers. Furthermore, many such galaxies appeared to be colliding with other galaxies (Plate 2.12). Could collisions of galaxies give birth to quasars, or refuel them? With its much greater sensitivity, SIRTF should be able to study the nature and evolution of these curious infrared galaxies. If a big fraction of quasars are produced by collisions of galaxies, SIRTF will make it possible for astronomers to find out.
Quasars and active galaxies emit copious amounts of power across a broad range of wavelengths, from radio waves to x-rays. Where does their great energy come from? It certainly cannot come entirely from stars that radiate predominantly visible light. Furthermore, stars live on nuclear energy, converting matter into energy with an efficiency of less than 0.1 percent. Nuclear energy is not efficient enough to balance the huge energy budget of quasars and active galaxies. Finally, the stars in a galaxy are scattered about, while the energy of active galaxies is produced in a highly concentrated region