Astronomers have recognized and cataloged spiral, elliptical, and irregular galaxies as far away as several hundred megaparsecs from Earth. Beyond this distance, galaxies appear so faint that it is difficult to discern their shapes. Consequently, their types are largely unknown. Nevertheless, according to their observed redshifts (and Hubble's law), we know that many galaxies lie well beyond this distance, in the farthest reaches of the observable universe. But what kinds of objects are they? Are they normal galaxies—close relatives of the galaxies that populate our local neighborhood—or are they somehow different? The answer is that, although most distant galaxies appear basically normal in their properties, some are distinctly different from galaxies found in our cosmic backyard: they are more luminous—more active—than the typical galaxy found close to home. These energetic objects, some of them hundreds, even thousands, of times more luminous than the Milky Way Galaxy, are known collectively as active galaxies.

In addition to their greater overall luminosities, active galaxies differ fundamentally from normal galaxies in the character of the radiation they emit. Most of a normal galaxy's radiated energy is emitted in or near the visible portion of the electromagnetic spectrum, much like the radiation from ordinary stars. Indeed, to a large extent, the light we see from a normal galaxy is just the accumulated light of its many component stars. For example, our entire Milky Way has a luminosity of about 10³⁷ W at optical frequencies—20 billion Suns' worth of radiation—but only 10³¹ W at radio frequencies—a million times less. By contrast, as illustrated in Figure 25.1, the radiation observed emitted by active galaxies does not peak at optical frequencies—far more energy is emitted at longer wavelengths than in the visible range. The radiation from active galaxies is inconsistent with what we would expect if it were the combined radiation of myriad stars. The radiation is thus said to be nonstellar in nature.

Figure 25.1 The nature of the energy emitted from a normal galaxy differs from that of an active galaxy. (This plot illustrates the general run of intensity for all galaxies of a particular type and does not represent any one individual galaxy.)

The two most important categories of active galaxies are Seyfert galaxies and radio galaxies, although other classes exist (see, for example, Interlude 25-1). Even more extreme in their properties are the quasars. Astronomers conventionally distinguish between active galaxies and quasars based on their spectra, appearance, and distance from us. At visible wavelengths active galaxies often look like fairly normal galaxies. Indeed, for many purposes, we can think of active galaxies as being otherwise "normal" systems (that is, emitting visible radiation) that happen also to be extremely intense sources of radio and/or infrared radiation. Quasars, on the other hand, are mostly so far away that little internal structure can be discerned. However, this distinction between active galaxies and quasars is largely historical, dating from the days when the connection between them was not understood. As we will see, most astronomers now believe that quasars are simply an early stage of galaxy formation and that there is really no sharp dividing line between quasars and active galaxies. Many researchers include quasars in the "active galaxy" category.

Not all active galaxies are distant, and only a small fraction of all distant galaxies are active. Some active galaxies are found locally, scattered among the normal galaxies that make up most of our cosmic neighborhood, and many faraway "normal luminosity" galaxies are known. As a general rule, though, active galaxies are more common at greater distances, and the most active objects lie farthest from Earth.

We might wonder whether this predominance of energetic objects with large redshifts is just an observational effect, resulting from our inability to detect relatively faint normal galaxies at great distances. After all, the apparent brightness of any astronomical object decreases as the square of its distance from us, so even with the very best telescopes, we would expect to observe only the more energetic and powerful galaxies in remote regions of space. Although this observational bias does play a role, it turns out that it can only partly explain the apparent predominance of energetic objects among those having large redshifts. Bright active galaxies really are more common at great distances.

Physical conditions were undoubtedly different at earlier times from what they are now. Perhaps, then, we should not be surprised that remote astronomical objects, which long ago emitted the radiation we observe today, differ from nearby objects, which emitted their radiation much more recently. What is surprising—in fact, astounding—is the amount of energy radiated by some of the most luminous objects. Their tremendous power, nonstellar radiation, and abundance at great distances suggest to many astronomers that the universe was once a much more violent place than it is today.